1. Copy label into derived formats
2. If audio is the primary track type, copy additional fields from
the variant formats. This is probably a no-op in practice, since
I don't think variant formats have these fields set anyway, but
it seems like the right thing to do in case they ever are set in
the future.
Note: It's a bit strange to use createXContainerFormat rather than
createXSampleFormat, but in practice the methods used here are
better matched for what we're trying to do.
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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=204467287
