[webrtc-pc] Define what the default RTCDataChannel id is

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

== Define what the default RTCDataChannel id is ==
https://w3c.github.io/webrtc-pc/#dom-rtcpeerconnection-createdatachannel

> If the id attribute is uninitialized (not set via the dictionary), and the DTLS role of the SCTP transport has already been negotiated, then initialize id to a value generated by the user agent, according to [RTCWEB-DATA-PROTOCOL], and skip to the next step. If no available id could be generated, or if the value of the id member of the dictionary is taken by an existing RTCDataChannel , throw a ResourceInUse exception.

However, Chrome and Firefox seem to be doing something much simpler, for `new RTCPeerConnection().createDataChannel('').id` both simply return 65535.

Please view or discuss this issue at https://github.com/w3c/webrtc-pc/issues/1132 using your GitHub account