[webrtc-pc] We forgot about {offerToReceiveAudio: false, offerToReceiveVideo: false}

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

== We forgot about {offerToReceiveAudio: false, offerToReceiveVideo: false} ==
The legacy [offerToReceive](http://w3c.github.io/webrtc-pc/#dom-rtcofferoptions-offertoreceiveaudio) section only covers how we used to set `"recvonly"`:
```js
pc.createOffer({offerToReceiveAudio: true});
```
but not how we used to set `"sendonly"`:
```js
pc.addStream(stream);
pc.createOffer({offerToReceiveAudio: false});
```

Try it with this [fiddle](https://jsfiddle.net/jib1/1h8mr8fw/). Was this omitted on purpose?

Having it implicitly call `setDirection("sendonly")` might suffice.

Please view or discuss this issue at https://github.com/w3c/webrtc-pc/issues/1461 using your GitHub account