They don't really belong there; it was basically a convenience
thing where one of the arguments to the track selector was being
packaged up in the result to avoid having to hold a separate
reference to it.
This change is being made as a precursor to a subsequent change
where creating the TrackSelectorResult will move from
MappingTrackSelector to DefaultTrackSelector. DefaultTrackSelector
doesn't currently have access to the un-mapped tracks, and so is
unable to create a TrackSelectorResult. It's IMO preferable to
keep it that way rather than passing them down just so they can
be included in the result.
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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=190640594
