MatroskaExtractor was checking for INDEX_UNSET as the sample count for buffers
not containing syncframes, but actually 0 was returned for these. Theoretically
this could prevent us starting to play a TrueHD stream as we wait until we can
read the sample count from a syncframe before accepting the audio in
DefaultAudioSink, but it seems that rechunking avoided this issue arising in
practice.
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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=206575462
