[webrtc-pc] Pull Request: Set direction="inactive" if rejected

jan-ivar has just submitted a new pull request for https://github.com/w3c/webrtc-pc:

== Set direction="inactive" if rejected ==
Fixes https://github.com/w3c/webrtc-pc/issues/1812.

This treats *direction* as `"inactive"` in SRD(offer) if m-section is rejected.

See https://github.com/w3c/webrtc-pc/pull/2118