This maps, in both directions, [Audio/Video]TrackInterface with
[Voice/Video][Sender/Receiver]Info.
This mapping is necessary for RTCStatsCollector to know the relationship
between RTCMediaStreamTrackStats and RTC[In/Out]boundRTPStreamStats, and
to be able to collect several RTCMediaStreamTrackStats stats.
BUG=webrtc:6757, chromium:659137, chromium:657854, chromium:627816
Review-Url: https://codereview.webrtc.org/2611983002
Cr-Commit-Position: refs/heads/master@{#16090}
