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Before starting decoding, MojoVD asks which configs are supported by
GPU. Currently we still get the config via VDA implementations even we
enable new VD.

This CL adds new method "GetSupportedVideoDecoderConfigs" at V4L2SVD
and VaapiVD, and query the supported configs from them.

BUG=chromium:952730
TEST=Check the supported video decoder configs are the same
     on Kevin(V4L2) and Eve(Vaapi)

Change-Id: I12ad2662e2c5462878d7f74f9541906946b5d9c3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1672852
Commit-Queue: Chih-Yu Huang <akahuang@chromium.org>
Reviewed-by: Dan Sanders <sandersd@chromium.org>
Reviewed-by: David Staessens <dstaessens@chromium.org>
Cr-Commit-Position: refs/heads/master@{#672782}