[webrtc-stats] Pull Request: Adding audio level stat that can be used to compute averages.

taylor-b has just submitted a new pull request for https://github.com/w3c/webrtc-stats:

== Adding audio level stat that can be used to compute averages. ==
Fixes #220.

It's slightly different than most of the stats of this sort, since the
application needs to compute "sqrt(a)/b", not just "a/b". But the
concept is the same.

See https://github.com/w3c/webrtc-stats/pull/221