Set / verify stats report timestamps.

This CL updates the track report timestamps which were fixed at "0" before
and updates the timestamps in reports for local audio tracks.

Also the timestamps are checked in various tests to make sure no "0" is
returned.

Original CL is at https://webrtc-codereview.appspot.com/51829004/

BUG=webrtc:4316
TBR=hta@webrtc.org, pthatcher@webrtc.org

Review URL: https://codereview.webrtc.org/1204493002

Cr-Commit-Position: refs/heads/master@{#9485}
5 files changed