Use iceConnectionState to detect initial connection.

Issue #433
> It looks like onSignallingStateChange_ will still call completeConnection_ when it thinks it's connected.

I don't see it.  The only method calls I see are `this.close()` and `this.closeWithError_(...)`

> do you think we can remove onSignallingStateChange_ altogether

There is at least one situation (system suspend) where the browser can unexpectedly close your RTCPeerConnection, so I think listening to signaling state changes is probably still a good idea.