RTCMediaStreamTrackStats collected on a per-attachment basis.

According to recent spec change:
https://github.com/w3c/webrtc-stats/pull/138/files

This establishes the relationship between tracks and
[Voice/Video][Sender/Receiver]Info(s). Follow-up CLs will easily be able
to collect more stats from them.

BUG=webrtc:6757, chromium:659137, chromium:627816

Review-Url: https://codereview.webrtc.org/2641763003
Cr-Original-Commit-Position: refs/heads/master@{#16181}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 9e30274c03c8e812abeb37c7876648e3cae0805a