[WIP] Allow running macOS embedder with UI and platform thread merged

[knopp]

Original issue: flutter/flutter#150525

This is a work in progress PR for letting the macOS embedder run both with and without UI and platform thread merged.

Changes in the resize / vsync synchronization:

• Added FlutterRunLoop class to schedule tasks Flutter tasks on main thread in a way where it is possible to only process these tasks while waiting for correct frame size during resizing. This significantly simplifies the resize synchronization and makes the same code work both with separate UI thread and with UI and platform thread merged.
• FlutterThreadSynchronizer has been renamed to FlutterResizeSynchronizer vastly simplified, mutex and conditions are removed and the blocking is now done by only processing Flutter messages while waiting for resizing. It is now per view (instead of storing a viewId->Size map internally) and owned by the view itself, instead of engine.

Regarding resize synchronization this approach should work for Windows and Linux embedders as well. On both platforms we manage the task queue internally and are able to only process only Flutter messages so that we can block the platform thread while we keep the UI task runner going.

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the Flutter Style Guide and the C++, Objective-C, Java style guides.
• I listed at least one issue that this PR fixes in the description above.
• I added new tests to check the change I am making or feature I am adding, or the PR is test-exempt. See testing the engine for instructions on writing and running engine tests.
• I updated/added relevant documentation (doc comments with ///).
• I signed the CLA.
• All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel on Discord.

[knopp]

Remaining issues to solve:

• Tests need to be updated
• Configure through Info.plist
• Figure out how to flush the microtask queue

The microtask queue is currently being flushed though a MessageLoopTaskQueues observers, which is being setup in a slightly convoluted way through callbacks in DartIsolate constructor. This works for iOS and Android since the message loop for platform thread exists on those (and is compatible with platform thread because it uses CFRunLoop and ALooper) internally, but this is not the case on desktop in general (it does not exist on platform thread and even if it did, it would not be compatible on Windows and Linux).

Instead, we should post the message directly the task runner provided by the embedder, and possibly flush the microtask queue after each task internally.

cc @jason-simmons, @jonahwilliams

[flutter-dashboard]

This pull request executed golden file tests, but it has not been updated in a while (20+ days). Test results from Gold expire after as many days, so this pull request will need to be updated with a fresh commit in order to get results from Gold.

[chinmaygarde]

Since this is blocked on the linked issue, can we close this till support for that API is added?