*** Original change description ***
Pull assertion and layer of indirection out from ChunkExtractorWrapper
It should be possible to remove ChunkExtractorWrapper from the track
output side as well (currently all extractor output is funneled via
ChunkExtractorWrapper just so it can adjust the format, which is kind
of unnecessary).
***
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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=145078094
5407c98526