[webrtc-pc] Pull Request: non-nullable RTCTrackEvent args means Init dict members are required.

jan-ivar has just submitted a new pull request for 
https://github.com/w3c/webrtc-pc:

== non-nullable RTCTrackEvent args means Init dict members are 
required. ==
Mozilla's webidl compiler actually requires this or the binding code 
wont compile (it's undefined what happens when no args are passed). 
It's either this or make the RTCTrackEvent args nullable and default 
to `null`.

See https://github.com/w3c/webrtc-pc/pull/446