[webrtc-stats] new commits pushed by vr000m

The following commits were just pushed by vr000m to https://github.com/w3c/webrtc-stats:

* Adapt references to dictionary member existence (#557)

[WebIDL no longer defines a dictionary member as present](https://github.com/heycam/webidl/commit/8d4e7ca67bcb4de476aab0411531dc97f6d17e0d) but instead makes use of [map existence](https://infra.spec.whatwg.org/#map-exists) as terminology. The spec is updated to make a similar usage.
  by Dominique Hazael-Massieux
https://github.com/w3c/webrtc-stats/commit/8421af165d670b2b1b94b4178a8ff408dd94f77d