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This introduces GpuMemoryBufferDecoderWrapper which is used to wrap
software VideoDecoder implementations. The wrapper copies frames as
they are returned from the decoder, but before they are delivered to
the renderer.

This is an improvement over the current copy done by VideoRendererImpl
because the VideoFrameStream (which sits between the renderer and the
decoder) may hold undelivered frames until a subsequent Read(). At
which point the frame still needs to be copied.

Total reduction in read times is ~12.6% (VP9) to ~30.1% (H264) after
4000 frames when tested using local caches of:
https://www.youtube.com/watch?v=1La4QzGeaaQ (4K60 VP9)
https://www.youtube.com/watch?v=dEaSepKi9SQ (4K60 H264)

The real benefit of this approach will come later when we allow
parallel decodes for offloaded decoders; since these decoders may
accumulate more frames in the decoder stream without this approach.

This also allows us to remove a bit of code and specialized tests
from the VideoRendererImpl in favor of more compartmentalized tests
just for the wrapper.

The downside to this approach is each decoder has it's own pool,
but this only amounts to ~240 bytes of memory when no frames
have been created.

Perf bots may detect a memory increase from this change, but only
as an artifact of improved throughput and timing changes.

BUG=801245
TEST=new unittests, manual tests.

Cq-Include-Trybots: master.tryserver.chromium.android:android_optional_gpu_tests_rel;master.tryserver.chromium.linux:linux_optional_gpu_tests_rel;master.tryserver.chromium.mac:mac_optional_gpu_tests_rel;master.tryserver.chromium.win:win_optional_gpu_tests_rel
Change-Id: Iac5655c9da3e850818f9de83cb2993358fcbd716
Reviewed-on: https://chromium-review.googlesource.com/947383
Commit-Queue: Dale Curtis <dalecurtis@chromium.org>
Reviewed-by: Daniele Castagna <dcastagna@chromium.org>
Cr-Commit-Position: refs/heads/master@{#541284}