[webrtc-pc] pc.createOffer(); pc.addTrack(track); should not include track

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

== pc.createOffer(); pc.addTrack(track); should not include track ==
Now that [createOffer has procedural 
steps](https://rawgit.com/w3c/webrtc-pc/7f66138784d86bf3f944a7327386ca0b10e351a9/webrtc.html#dom-rtcpeerconnection-createoffer)
 thanks to @adam-be, we should add language to capture some state 
synchronously, so that in the infamous case of:
```js
    pc.addTrack(trackX, stream);
    pc.createOffer().then(offer => { ... })
    pc.addTrack(trackY, stream);
```

the `offer` does not include `trackY` in the typical (nothing on the 
queue) case, as we agreed over a year ago.

Please view or discuss this issue at 
https://github.com/w3c/webrtc-pc/issues/782 using your GitHub account