[webrtc-pc] Don't fire events on a closed peer connection

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

== Don't fire events on a closed peer connection ==
To avoid this, whenever we *"Queue a task to run the following 
steps:"*, we immediately do:

 * If connection's [[isClosed]] slot is true, abort these steps.

We've adopted this approach, but we're not consistent. I found at 
least a couple of places where we don't do this:

 * The negotiationneeded event (we check [[needNegotiation]], but not 
[[isClosed]] 
[here](https://w3c.github.io/webrtc-pc/#dfn-update-the-negotiation-needed-flag)).
 * Resolve promise in 
[replaceTrack](https://w3c.github.io/webrtc-pc/#dom-rtcrtpsender-replacetrack).

There may be more places.

Please view or discuss this issue at 
https://github.com/w3c/webrtc-pc/issues/1020 using your GitHub account