[webrtc-pc] localDescription, remoteDescription et al. need internal slots

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

== localDescription, remoteDescription et al. need internal slots ==
Both Chrome and Firefox fail a basic test of:
```js
  await pc1.setLocalDescription(await pc1.createOffer());
  console.log(pc1.localDescription == pc1.localDescription); // false; should be true!
```
That's because we're returning a new instance on getting. Instead, we (and the spec) should return the value of an internal slot of type `RTCSessionDescription`.

Please view or discuss this issue at https://github.com/w3c/webrtc-pc/issues/1871 using your GitHub account