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When prerolling happens, MediaDecoderJob currently rolls to the timestamp from the access unit.
However, for video, the access unit's timestamp may not match that from the decoder due to reordering.
Even worse, the unit's timestamp may be greatly ahead of the presentation timestamp from the decoder since the decoder is still decoding previous access units. Some of the unit tests actually show this problem.

Fixed all the problematic unit tests along with this CL.

BUG=310823

Review URL: https://codereview.chromium.org/113963005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@243210 0039d316-1c4b-4281-b951-d872f2087c98