[webrtc-pc] RTCIceTransport role before start() called (#2210)

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

== RTCIceTransport role before start() called ==
This was first noticed in the webrtc-ice repository: https://github.com/w3c/webrtc-ice/issues/4

Before a role is assigned, it seems misleading to give "role" either of the values "controlling" or "controlled".

I suggest we create a new enum value called "unknown", which is the initial value.
(This name also corresponds to the implementation inside the WebRTC library. No need to make new names when we don't have to.)

Please view or discuss this issue at https://github.com/w3c/webrtc-pc/issues/2210 using your GitHub account