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To prevent potentially regressing legacy buffering-by-DTS MSE apps that
may deliberately provide incorrect video keyframeness metadata in their MP4,
this change preserves trust in that metadata when MseBufferByPts is not
enabled. If MseBufferByPts is enabled, this change preserves the recently
added behavior where video frame analysis results' keyframeness (if
implemented and determined) is used instead of the MP4's metadata.

BUG=892365
R=sandersd@chromium.org
TEST=MP4StreamParserTest.*AVC*Mismatches*, also manually confirmed
SPS/PPS injection/lack of injection matches the expected sequence of
Parsed{Non}Keyframe in those tests.

Change-Id: If756e0ee882ec17c0d13d7b400aa43626cedaca9
Reviewed-on: https://chromium-review.googlesource.com/c/1263426
Commit-Queue: Matthew Wolenetz <wolenetz@chromium.org>
Reviewed-by: Dan Sanders <sandersd@chromium.org>
Cr-Commit-Position: refs/heads/master@{#596954}