getStats in WebRTC

The WebRTC getStats API is specified in and allows querying information about the current state of a RTCPeerConnection API and some of its member objects.

Adding new statistics to Chrome

When adding a new standardized RTCStatsMember it is necessary to add it to the Chrome allowlist chrome/test/data/webrtc/peerconnection_getstats.js before landing the WebRTC change. This mechanism prevents the accidential addition and exposure of non-standard attributes and is not required for RTCNonStandardStatsMember which is not exposed to the web API.