When determining the next sample to load, the Mp4Extractor now takes
into account how far one stream is reading ahead of the others.
If one stream is reading ahead more than a threshold (default: 10 seconds),
the extractor continues reading the other stream even though it needs
to reload the source at a new position.
GitHub:#3481
GitHub:#3214
GitHub:#3670
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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=182504396