[webrtc-stats] Flatten the stats hierarchy (#533)

henbos has just created a new issue for https://github.com/w3c/webrtc-stats:

== Flatten the stats hierarchy ==
"track" is now obsolete and "sender" and "receiver" stats contain very few metrics.

Please view or discuss this issue at https://github.com/w3c/webrtc-stats/issues/533 using your GitHub account