This adds the getter function of the bit depth of a stream
to AcceleratedVideoDecoder interface.

Bug: b:172223934
Test: media_unittests
Test: video.DecodeAccelVD.* on trogdor
Change-Id: Id3a5a3f84736a64d229366c0f352272483fae4c6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2562798
Commit-Queue: Hirokazu Honda <hiroh@chromium.org>
Reviewed-by: Andres Calderon Jaramillo <andrescj@chromium.org>
Reviewed-by: Dale Curtis <dalecurtis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#833169}