ongatheringstatechange references wrong ICE gathering state

[jan-ivar]

RTCIceTransport's ongatheringstatechange accidentally references the RTCPeerConnection's ICE gathering state, calling it "RTCIceTransport's".

A while back, we converted most state to internal slots, except these four: signaling state, connection state, ice gathering state, and ICE connection state.

It'd be nice to finish this cleanup, to avoid further accidents of this kind.