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Using DecoderBuffer removes the need to provided SharedMemoryHandles in
cases that SharedMemory is not already allocated (in particular, when
VDAs are used through MojoVideoDecoder+VdaVideoDecoder). Since
base::SharedMemory is not an interface, a new type is required, and
DecoderBuffer already implements a suitable interface for this use case.

This CL adds:
  - A VideoDecodeAccelerator::Decode() overload for decoding
    DecoderBuffers, which VDAs may optionally implement.
    Note that VdaVideoDecoder will require this, and therefore all VDAs
    will be updated to support DecoderBuffers over time.
  - Support for SHM in DecoderBuffer. This eases the transition by
    allowing BitstreamBuffers to be easily converted to equivalent
    DecoderBuffers.
  - An implementation of the new VideoDecodeAccelerator::Decode()
    overload in VTVideoDecodeAccelerator.
  - Calling the new overload from VdaVideoDecoder.

Bug: 522298
Cq-Include-Trybots: luci.chromium.try:android_optional_gpu_tests_rel;luci.chromium.try:linux_optional_gpu_tests_rel;luci.chromium.try:mac_optional_gpu_tests_rel;luci.chromium.try:win_optional_gpu_tests_rel
Change-Id: Ib932c3524b37e339f592b150685937ea94d4199d
Reviewed-on: https://chromium-review.googlesource.com/1015965Reviewed-by: Pawel Osciak <posciak@chromium.org>
Reviewed-by: Dale Curtis <dalecurtis@chromium.org>
Commit-Queue: Dan Sanders <sandersd@chromium.org>
Cr-Commit-Position: refs/heads/master@{#553364}