[webrtc-pc] Data channel receive procedure missing?

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

== Data channel receive procedure missing? ==
It seems there is no data channel receive procedure that can be followed, e.g. nothing tells us when a *message* event is invoked and what validation steps are required (check for *open*, ...).

The WPT tests reference [this WS spec section](https://html.spec.whatwg.org/multipage/web-sockets.html#feedback-from-the-protocol) (*When a WebSocket message has been received [...]*) and I would propose to copy that.

Please view or discuss this issue at https://github.com/w3c/webrtc-pc/issues/1825 using your GitHub account