ChunkSampleStream.seekToUs assumes that if we can seek within the
primary sample queue, we can also seek within the embedded queues.
This assumption can be violated fairly easily if discardBuffer is
called with toKeyframe=true, since this can cause samples to be
discarded from the embedded queues within the period for which a
seek in the primary sample queue will succeed. This change fixes
the issue.
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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=181965902
