SDK / exoplayer

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

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=146471082