[webrtc-pc] enqueue an operation: is executing async?

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

== enqueue an operation: is executing async? ==
step 5 of [enqueue an operation](http://w3c.github.io/webrtc-pc/#enqueue-an-operation) says
> If the length of [[Operations]] is exactly 1, execute operation.

Is this synchronous or async? In terms of code (see [fiddle](https://jsfiddle.net/dgvdg7xj/1/)):
```
var pc = new RTCPeerConnection();
navigator.mediaDevices.getUserMedia({audio: true})
.then(stream => {
  var p = pc.createOffer();
  pc.addTrack(stream.getTracks()[0], stream);
  return p;
})
.then(offer => {
  console.log(offer.sdp);
})
```
Does offer have an m-line? Currently the answer in Chrome is "no" and in Firefox it is "yes".

editorial drive-by nit: the paragraph above that section that describes the operations queue should be placed inside the section


Please view or discuss this issue at https://github.com/w3c/webrtc-pc/issues/1711 using your GitHub account