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This CL migrates MailboxVideoFrameConverter from using Textures to
using SharedImages.  No external functionality changes are expected.

Couple of things need to be added:

- ScopedSharedImage: an internal class to scope a SharedImage lifetime.
 SharedImages are kept in |shared_images_|, indexed by a unique id.

- Added GetGpuChannelCB that can be derived from the GetStubCB and that
 is needed for SharedImage creation and destruction. They are generated
 on Create(), so this interface doesn't change.

- This CL uses a WeakPtr to the GpuChannel, accessing its SharedImageStub
 and the gpu::Scheduler that way.

Most of the work goes in:
 - InitializeOnGpuTaskRunner() and then
 - the sequence ConvertFrameOnGPUThread() (calls either
  GenerateSharedImageOnGPUThread() or UpdateSharedImageOnGPUThread()) then
  jumps back to WrapMailboxAndVideoFrameAndOutput().

Test: crosvideo.appspot.com resolution-changing videos  (vp9, h264, vp8)
of course with the new VD decoder, on kohaku and scarlet/dru.

Bug: 998279
Change-Id: Ic9b5b2270d1b488562bc2725bba89dc2a0350012
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1772642
Commit-Queue: Miguel Casas <mcasas@chromium.org>
Reviewed-by: Alexandre Courbot <acourbot@chromium.org>
Reviewed-by: Eric Karl <ericrk@chromium.org>
Reviewed-by: Chih-Yu Huang <akahuang@chromium.org>
Reviewed-by: Dan Sanders <sandersd@chromium.org>
Reviewed-by: Andres Calderon Jaramillo <andrescj@chromium.org>
Cr-Commit-Position: refs/heads/master@{#698666}