Add Bazel config for building CEF binary distribution

[magreenblatt]

Is your feature request related to a problem? Please describe.
Bazel is a popular alternative to CMake for building applications. We should provide sample configurations for Bazel as part of the CEF binary distribution.

Describe the solution you'd like
Build the CEF sample apps in a binary distribution using Bazel and the default platform toolchain.

Additional context
Bazel default platform toolchains are tested to work for Windows x64, MacOS ARM64 and x64 (cross-compile from ARM64), and Linux x64. They are known to be broken for Windows x86 (cross-compile from x64). Other platforms/architectures are untested and may require custom toolchain definitions (Linux examples here).

[magreenblatt]

Initial Bazel support is considered complete (working) in version 127.1.5+g35f74cc+chromium-127.0.6533.89.

Building with Bazel works as follows:

1. Install Bazelisk.
2. Download/extract a CEF binary distribution that includes Bazel support.
3. Build and run using Bazel:

$ cd /path/to/cef_binary_*
$ bazel build //tests/cefclient 

# On Mac/Linux:
$ bazel run //tests/cefclient

# On Windows:
$ bazel run //tests/cefclient/win:cefclient.exe

Platform-specific instructions are included in the binary distribution README.txt file.

See cef-project for an example Bazel project that utilizes the CEF binary distribution as an external dependency.

Notes:

• Add -c dbg for a Debug build
• Add --cpu=darwin_x86_64 for a Mac x64 cross-compile build on ARM64 host
• Arguments can be passed to the app when using run by adding -- [...] at the end

[magreenblatt]

In-progress PR: https://bitbucket.org/chromiumembedded/cef/pull-requests/786

[magreenblatt]

Remaining work:

• (DONE) Figure out how to consume the CEF binary distribution (as repository or module) from a third-party Bazel project. Update cef-project to demo this.
• Windows: Add configuration for USE_ATL.

Related to C++ compiler/toolchain configuration:

• Determine if additional compiler flags should be added in bazel/[win|mac|linux]/variables.bzl, or if we should leave those up to the toolchain.
• Consider adding a REPO.bazel file (design doc).
• Linux: Add per-compiler/version/feature config, maybe as flags in .bazelrc.
• Linux: Maybe find a a better way to solve rpath configuration so we can remove the patchelf package dependency.
• Windows: Figure out how PDB files are supposed to work with opt builds (see here).

[magreenblatt]

Figure out how to consume the CEF binary distribution (as repository or module) from a third-party Bazel project.

(For background see https://bazel.build/external/overview)

The http_archive rule allows us to download an archive file, and optionally override the associated BUILD and/or WORKSPACE file. We'll need a convenient way to collect download URLs and sha256 values for all platform builds of a particular release. For example, see #3757 (comment).

[magreenblatt]

We'll need a convenient way to collect download URLs and sha256 values

The Spotify builder now provides <file>.sha256 for all binaries beginning with version 127.1.2.

[magreenblatt]

The http_archive rule allows us to download an archive file, and optionally override the associated BUILD and/or WORKSPACE file.

Unfortunately this approach has a number of limitations. For example:

• We can download a platform/arch-specific archive as name="cef_<platform>" and create target-specific aliases from @cef to @cef_<platform> using select({}), but we can't create similar aliases for macros or rules (*.bzl files) included in the distribution. Consequently, there is no way to load("@cef//...", ...) and have it redirect to load("@cef_<platform>//...", ...). See Cannot load .bzl via alias label bazelbuild/bazel#10092 for background. This would require us to manually copy/commit the scripts from the distribution into the top-level client repository, or move them to a separate CEF artifact/repository (without platform/arch-specific name).
• We can't have select({}) evaluate to strings as input to WORKSPACE load-related rules/macros (e.g. for downloading a platform/arch-specific archive as name="cef"). See https://stackoverflow.com/a/71199926 for background.

We will therefore provide a separate script for downloading the CEF binary distribution and writing a version.bzl file, to be run manually before bazel build. The WORKSPACE configuration then becomes:

load(":third_party/cef/version.bzl", "CEF_VERSION", "CEF_PLATFORM")

local_repository(
    name = "cef",
    path = "third_party/cef/cef_binary_{}_{}".format(CEF_VERSION, CEF_PLATFORM),
)

[magreenblatt]

We can download a platform/arch-specific archive as name="cef_<platform>" and create target-specific aliases from @cef to @cef_<platform> using select({})

Here's an example of this approach for future reference:

## `WORKSPACE` contents:

local_repository(
    name = "cef",
    path = "bazel/cef",
)
load("@cef//:repos.bzl", "install_cef_repos")
install_cef_repos()


## `bazel/cef/BUILD.bazel` contents:

# Alias all targets to the platform/arch-specific repo.
alias(
    name = "cef_wrapper",
    actual = select({
        "@//:macos_arm64": "@cef_macosarm64//:cef_wrapper",
        # ...
    }),
)

# ...


## `bazel/cef/repos.bzl` contents:

load("@bazel_tools//tools/build_defs/repo:http.bzl", "http_archive")

CEF_DOWNLOAD_URL = "https://cef-builds.spotifycdn.com/"
CEF_VERSION = "127.1.2+g1cd2424+chromium-127.0.6533.73"

def add_cef_repo(
        platform,
        sha256,
        version = CEF_VERSION,
        url = CEF_DOWNLOAD_URL):
    file_name = "cef_binary_{}_{}".format(version, platform)
    http_archive(
        name = "cef_{0}".format(platform),
        sha256 = sha256,
        strip_prefix = file_name,
        url = "{}{}".format(url, file_name),
    )

def install_cef_repos():
    """
      Set up definitions of all repos for each supported platform/arch.
    """
    add_cef_repo(platform = "macosarm64", sha256 = "d2e9ead...")
    # ...

[magreenblatt]

We will therefore provide a separate script for downloading the CEF binary distribution

There's also a middle ground where the script writes the platform but Bazel still performs the download:

## `cef_config.bzl` contents (written by script):

CEF_PLATFORM = "macosarm64"


## `WORKSPACE` contents:

load("//:cef_config.bzl", "CEF_PLATFORM")
load("//:bazel/cef_loader.bzl", "load_cef_repo")
load_cef_repo(CEF_PLATFORM)


## `bazel/cef_loader.bzl` contents:

load("@bazel_tools//tools/build_defs/repo:http.bzl", "http_archive")

CEF_DOWNLOAD_URL = "https://cef-builds.spotifycdn.com/"
CEF_VERSION = "127.1.2+g1cd2424+chromium-127.0.6533.73"
CEF_FILE_SHA256 = {
    # Map of each platform name to sha256 hash.
    "macosarm64": "d2e9ead6...",
    # ...
}

def load_cef_repo(platform
                  name = "cef",
                  version = CEF_VERSION,
                  url = CEF_DOWNLOAD_URL):
    """
    Download, extract and load a CEF binary distribution.
    """
    file_name = "cef_binary_{}_{}".format(version, platform)

    http_archive(
        name = name,
        sha256 = CEF_FILE_SHA256[platform],
        strip_prefix = file_name,
        url = "{}{}.tar.bz2".format(url, file_name),
    )