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Currently RenderFrameMetadata is communicated from the renderer to the browser.
The messages are processed right away, even if Viz may have not yet completed
the processing of the frame associated to the data. This is racy.

This change adds a FrameToken to all of the RenderFrameMetadata messages. The
browser side then uses FrameTokenMessageQueue to synchronize these messages.
They will now be processed once Viz has completed processing the frame and has
notified the browser.

TEST=existing viz_content_browsertests
TBR=piman@chromium.org

Bug: 775103
Change-Id: I52e6d023be5591946c589400374236a2a8e979d6
Reviewed-on: https://chromium-review.googlesource.com/962711Reviewed-by: Jonathan Ross <jonross@chromium.org>
Reviewed-by: Greg Kerr <kerrnel@chromium.org>
Reviewed-by: Fady Samuel <fsamuel@chromium.org>
Commit-Queue: Jonathan Ross <jonross@chromium.org>
Cr-Commit-Position: refs/heads/master@{#544072}