[webrtc-pc] The RTCDataChannelEventInit channel member needs to be required

foolip has just created a new issue for 
https://github.com/w3c/webrtc-pc:

== The RTCDataChannelEventInit channel member needs to be required ==
https://w3c.github.io/webrtc-pc/#rtcdatachannelevent

If it isn't required, then `new RTCDataChannelEvent('type', {})` won't
 throw, and the resulting instance will have null for the `channel` 
attribute even though it's not nullable in the IDL. So:

```WebIDL
dictionary RTCDataChannelEventInit : EventInit {
    required RTCDataChannel channel;
};
```

Please view or discuss this issue at 
https://github.com/w3c/webrtc-pc/issues/817 using your GitHub account