SLD without {sdp} reads [[LastCreatedOffer/Answer]] at the wrong time

[jan-ivar]

The current prose on setLocalDescription reads [[LastCreatedOffer]] and [[LastCreatedAnswer]] on method entry. This is the wrong time, and may produce the wrong result if other methods are enqueued. For example, it would fail in the FIFO peer case:

io.onmessage = async ({data: {description, candidate}}) => {
  if (description) {
    await Promise.all([                           // ←- avoids race!
      pc.setRemoteDescription(description),       // ←- 
      pc.createAnswer(),                          // ←- 
      pc.setLocalDescription({type: "answer"})    // ←- unusual, works today
    ]);                                           // ←- 

Here, all three promise-returning negotiation methods are invoked at the same time, which means setLocalDescription would read [[LastCreatedAnswer]] before the createAnswer has succeeded (or even begun the in-parallel part of its algorithm). This would cause SLD to either fail (if [[LastCreatedAnswer]] was null, or apply an outdated answer.

Instead, we need to read these internal slots in the enqueued steps.