This class can accommodate extra information that may potentially
be specified by TrackSelectors in the future (e.g. configuration
for renderers). Even if we don't do this, it's cleaner to have a
proper class and an info field to refer to, as opposed to using a
Pair.
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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=144428963
