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This adds the VAAPI accelerator delegate for H265 decoding. This is not
actually enabled yet because there are other pieces higher up in the
decoding pipeline that still need to be turned on.

BUG=chromium:1141237,b:153111783
TEST=HEVC playback works w/ full set of changes, vaapi_unittest

Change-Id: Ic0a2dbe774267d4852e0d4d47aa378c6602ad41f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2530798Reviewed-by: Dale Curtis <dalecurtis@chromium.org>
Reviewed-by: Sergey Volk <servolk@chromium.org>
Reviewed-by: Miguel Casas <mcasas@chromium.org>
Commit-Queue: Jeffrey Kardatzke <jkardatzke@google.com>
Cr-Commit-Position: refs/heads/master@{#827050}