This allows binding by track type in ChunkExtractorWrapper, which
allows the EMSG and 608 tracks to be enabled on FragmentedMp4Extractor
in DefaultDashChunkSource. ChunkExtractorWrapper currently binds these
to DummyTrackOutputs.
Note: I wanted to pass the mimeType instead, since it's a more specific,
but unfortunately there's at least one place where it's not known at the
point of invoking track() (FlvExtractor).
Issue #2176
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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=146471082
