[webrtc-pc] pc.close() should null out currentDirection (#2122)

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

== pc.close() should null out currentDirection ==
[stop()](https://w3c.github.io/webrtc-pc/#dom-rtcrtptransceiver-stop) will *"Set transceiver's [[CurrentDirection]] slot to null."*, whereas [close()](https://w3c.github.io/webrtc-pc/#dom-rtcpeerconnection-close) won't. Omission.

Would be great to find a way for *close* to reuse the [stop the transceiver](https://w3c.github.io/webrtc-pc/#dfn-stop-the-rtcrtptransceiver) algorithm *stop* uses here. Unfortunately, they differ on firing the *ended* event.

Please view or discuss this issue at https://github.com/w3c/webrtc-pc/issues/2122 using your GitHub account