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CompositorFrameMetadata can arrive on the UI thread out of order:
Metadata is sent from compositor to UI in response to synchronous or
asynchronous draws. The synchronous draw replies is processed
immediately, even if there are replies to asynchronous draws pending. So
when there is a mix of sync and async draws, it's possible for metadata
to arrive out of order.

This has been a theoretical problem for a long time, but only became
real problem when FrameSwapMessageQueue was expecting metadata to be
ordered. The only practical fix is to simply drop out of order metadata
instead, which FrameSwapMessageQueue is designed to handle already.

Bug: 821706
Change-Id: I493862ba6d0838097f26e824359f03017eaa2a35
Reviewed-on: https://chromium-review.googlesource.com/985096Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Commit-Queue: Bo <boliu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#546750}