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This CL adds an implementation of AcceleratedVideoDecoder for
AV1, AV1Decoder and a few necessary classes for it. They use
libgav1 parser and structures. Since libgav1 is built in
ChromeOS, AV1Decoder is built for ChromeOS x86 targets only.

Bug: 1029212, 1000988
Test: video_decode_accelerator_tests test-25fps.av1.ivf --use_vd
Test: av1_decoder_unittest
Change-Id: Id9d0be18484006005803ae409e45c2028d44c315
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2397997
Commit-Queue: Hirokazu Honda <hiroh@chromium.org>
Reviewed-by: Andres Calderon Jaramillo <andrescj@chromium.org>
Cr-Commit-Position: refs/heads/master@{#827593}