[webrtc-pc] new commits pushed by aboba

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

* Make RTCErrorEvent's error member non-nullable

By making it required in the RTCErrorEventInit dictionary, it can never
be null and so can be made both non-nullable and marked [SameObject].
  by Philip Jägenstedt
https://github.com/w3c/webrtc-pc/commit/c35bed0b08e67c88f76ccb5e509fb4b03dbd8cc9

* Merge pull request #2099 from foolip/patch-3

Make RTCErrorEvent's error member non-nullable
  by Bernard Aboba
https://github.com/w3c/webrtc-pc/commit/6492ee087e6f6876d1b0af7bf45e9edf5c133d4d