[webrtc-stats] new commits pushed by alvestrand

The following commits were just pushed by alvestrand to https://github.com/w3c/webrtc-stats:

* Add "sender" and "receiver" stats. (#273)

* Split RTCMediaStreamTrackStats into four dictionaries.

* Add "sender" and "receiver" stats, as complement to "track" stats.

* Update RTCStatsType 'track' description.

* Two typos.

* Nits unrelated to the structural changes.

* s/video frame/frame/ and s/audio sample/sample/

* Remove now redundant "Only valid for video".

* Remove detached member.
  by jan-ivar
https://github.com/w3c/webrtc-stats/commit/ced70028d2f81b20c9eea84d5fe1703ffddfcf27