Currently FragmentedMp4Extractor only parses and outputs emsg messages if the
flag FLAG_ENABLE_EMSG_TRACK is set (when there's a metadata renderer that
handles emsg messages). Since there are emsg messages that only targets the
player, which we want to handle independently from MetadateRenderer, this CL
adds the ability for FragmentedMp4Extractor to output emsg messages to an
additional TrackOutput if provided, independently from FLAG_ENABLED_EMSG_TRACK.
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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=177318983
3afdb24f25