MSE: Use video (AVC) keyframeness when it differs from MP4's

Due to prevalence of MSE support in other browsers for legacy
incorrectly muxed content where AVC keyframeness is incorrectly marked
in the MP4 container, this change expands the previous
chrome://media-internals logging when such mismatch is detected, to
trust the keyframe-ness of the AVC coded frame for buffering and
decoding.  Otherwise, such streams, especially those with frames
incorrectly marked as keyframes in the MP4, would be buffered as
out-of-order GOPS rather than out-of-order nonkeyframes, corrupting the
buffering and decode sequence on playback in the new, compliant,
MseBufferByPts logic.  Seek preroll from actual AVC nonkeyframes in
legacy MseBufferByDts logic should also be fixed by this change.

BUG=879734,844799,229412,760264

Change-Id: I90f53ec333e1aeeeaf39e96e176438cb13b4a195
Reviewed-on: https://chromium-review.googlesource.com/1205640
Commit-Queue: Matthew Wolenetz <wolenetz@chromium.org>
Reviewed-by: Dale Curtis <dalecurtis@chromium.org>
Reviewed-by: Dan Sanders <sandersd@chromium.org>
Cr-Commit-Position: refs/heads/master@{#589009}
2 files changed