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This adds VdaVideoDecoder, an adapter that implements the media::VideoDecoder interface using a
media::VideoDecodeAccelerator. VdaVideoDecoder expects to run in the GPU process, with access
to a command buffer stub, as will be the case for decoders created by MojoVideoDecoder.

VdaVideoDecoder runs on the (mojo) IO thread as much as possible, but the VDA interface is
inherently tied to the GPU thread.

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:android_optional_gpu_tests_rel;luci.chromium.try:linux_optional_gpu_tests_rel;luci.chromium.try:mac_optional_gpu_tests_rel;master.tryserver.chromium.win:win_optional_gpu_tests_rel
Change-Id: I23f81c92b9ad72a5f0141c2eec2a528de1ffa9d3
Bug: 522298
Reviewed-on: https://chromium-review.googlesource.com/940336Reviewed-by: Xiaohan Wang <xhwang@chromium.org>
Reviewed-by: Frank Liberato <liberato@chromium.org>
Commit-Queue: Dan Sanders <sandersd@chromium.org>
Cr-Commit-Position: refs/heads/master@{#550471}