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This CL adds the ability to clone a blink::VideoFrame. Doing so creates
a new VideoFrame, with a new reference to the same underlying
media::VideoFrame. This allows users that want to keep a frame alive
to pass a clone to workers or objects which might destroy the frame.

An example use case might be a user sending cloned reference to
different workers, which can then call destroy() on the frames without
impacting each other's work. A user could also send a clone to
VideoEncoder, to keep using the frame after the VideoEncoder is done
with the clone and calls destroy on it.

Bug: 1108023
Change-Id: Ia4f22f1429e466474fb87ce9700a54b87428440b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2355092
Commit-Queue: Thomas Guilbert <tguilbert@chromium.org>
Reviewed-by: Dan Sanders <sandersd@chromium.org>
Auto-Submit: Thomas Guilbert <tguilbert@chromium.org>
Cr-Commit-Position: refs/heads/master@{#798386}