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This CL ensures:
- VideoFrame mailboxes are imported using the
  CreateAndTexStorage2DSharedImage API if they are shared images, and
  that the Begin/EndSharedImageAccessDirect scope are properly used.
- Intermediate copies are allocated as shared images instead of
  internally to skia.

The cache is modified to track the mailbox for the source texture,
rather than a SkImage, to be able to import it into multiple contexts.
Also the cached shared image is reused if possible (same context
provider, same size) to reflect equivalent skia optimizations (SkImage
pooling).

Note: this CL doesn't cover PrepareVideoFrameForWebGL, used for the
(still experimental) WEBGL_video_texture extension, which requires a
fairly significant refactoring all the way to WebGL bindings to support
shared image.

Bug: 882547
Change-Id: I77b374526f3617db0fd936ffa5b71885e2c8fab3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1616978
Commit-Queue: Antoine Labour <piman@chromium.org>
Reviewed-by: Dan Sanders <sandersd@chromium.org>
Auto-Submit: Antoine Labour <piman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#680021}