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This CL adds a new callback to VideoFrameCompositor. A client can now
set a callback, to be notified whenever a new frame has been presented
for composition.

This will be used to ultimately fulfill callbacks registered through the
new <video>.requestAnimationFrame API.

Bug: 1012063
Change-Id: If7c9e4192db03b81a9040219bedec70e38241509
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1887743
Commit-Queue: Thomas Guilbert <tguilbert@chromium.org>
Reviewed-by: Dale Curtis <dalecurtis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#710641}