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Preparing to implement WebCodecs (https://www.chromestatus.com/feature/5669293909868544).

Also add an impl of VideoFrame and a skeleton class of VideoDecoder.

Bug: 897297
Change-Id: I722ef891c10795e815ce0772b6f1be3afc563d6f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1946940
Commit-Queue: Peter Thatcher <pthatcher@google.com>
Reviewed-by: Mounir Lamouri <mlamouri@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Dan Sanders <sandersd@chromium.org>
Cr-Commit-Position: refs/heads/master@{#722564}