browser(webkit): let web page close when it has open context menu

[aslushnikov]

Review URL: aslushnikov/WebKit@42f86e9

Currently, if web page has an open context menu, then it won't close.
This prevents browser from quitting.

In stock safari, this behavior can also be observed in a way that
context menu will stay opened even if related page got closed.

While investigating this behavior on Mac, a crucial observation was
that [NSMenu popUpContextMenu] is spawning a nested event loop,
keeping reference to WebContextMenuProxyMac instance, which in turn
keeps references on associated NSView with WKWebView.

To exit the loop, we need to explicitly cancel context menu. For this,
this patch adds a method hide on WebContextMenuProxy that uses
port-specific code to cancel context menu.

Windows part of this patch is somewhat speculative: I didn't check
it, but given the same symptomps, I applied the same solution.

Fixes #2700