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Move current Chromium H264 accelerator to legacy and add support for
new H264 headers.
The use of new or legacy H264 accelerator is decided by whether the driver
supports request API or not.

BUG=chromium:987856
TEST=VDA unittest with test-25fps.h264 and frame validation on
     Minnie 3.14 and Kevin 4.4.

Change-Id: I103a0a8d08171ceb96f020c08125686d874ee41d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1723332Reviewed-by: Alexandre Courbot <acourbot@chromium.org>
Commit-Queue: Alexandre Courbot <acourbot@chromium.org>
Cr-Commit-Position: refs/heads/master@{#685472}