The track-level stats are currently implemented in terms of the stream-
level stats. Which is a problem if multiple unsignaled streams map to the
same track (see bug for more details). This CL fixes the problem
partially, but only returning stats for one of the unsignaled streams.
A better solution would be to return stats for both streams, but update
the track-level stats independently somehow. But that would require more
extensive changes, and it's not yet clear how we want to do it.
BUG=webrtc:8158
Review-Url: https://codereview.webrtc.org/3008373002
Cr-Commit-Position: refs/heads/master@{#19907}
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