Shell::OnPlatformViewCreated should not serialize on the UI thread

[dnfield]

This is a snip from a trace during Shell::OnCreatePlatformView. The highlighted section is where the call is running on the platform thread, and the picture is showing what's going on on the UI thread:

We should not be blocking progress on the platform thread while the UI thread is doing this work, and although the comment here says that this is important for some platforms, I'm not convinced that is true.

@chinmaygarde pointed out that the ordering is important in the OnPlatformViewDestroyed route, which I agree with and am less concerned about anyway. A couple concerns that he raised:

• Will this interrupt ordering of OnPlatformViewDestroyed? No, because a platform task cannot get scheduled before we've enqueued the raster/UI work, and the raster/UI work it enqueues will be blocked by the tasks we've enqueued.
• Will the logic to check whether the raster/platform thread have been merged still be valid? Yes, because the UI thread will not get a chance to create a platform view, which would require time on the platform runner, before we use that variable.

[dnfield]

/cc @gaaclarke who may have ideas about what this could break on iOS. /cc @blasten who probably has some more paged in for Android.

[dnfield]

From chat, a test case Chinmay mentioned we should have for this:

In shell_unittests, create a shell that calls OnPlatformViewCreated, invoke main that calls back into native code to wait for a latch. This will block the UI thread. Then call PlatformViewDestroyed and release the latch. In dart code, the add a platform view and try to render a frame with it. See what happens?

[github-actions]

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