[webrtc-pc] Pull Request: Make RTCErrorEvent's error member non-nullable

foolip has just submitted a new pull request for 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].

See https://github.com/w3c/webrtc-pc/pull/2099