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This enables VaapiVEA to encode DMABuf-backed video frame. Because the
va surface format is NV12, NV12 format is only acceptable.
This change is tested by crrev.com/c/1295636

BUG=chromium:895230
TEST=VEA unittest --native_input

Change-Id: Ib3c09363936cdf3099bb9ed9fc14eb24bd0e70fe
Reviewed-on: https://chromium-review.googlesource.com/c/1306944
Commit-Queue: Hirokazu Honda <hiroh@chromium.org>
Reviewed-by: Miguel Casas <mcasas@chromium.org>
Reviewed-by: Pawel Osciak <posciak@chromium.org>
Cr-Commit-Position: refs/heads/master@{#606519}