Fix RTCSession.get(Local|Remote)Streams in FF 22

In FF 22 localStream and remoteStreams on mozRTCPeerConnection are properties and the code to add methods getters on mozRTCPeerConnection for them doesn't work. It seems the object is not a 100% Javascript object, as it lacks prototype and attempts to add it don't work, either.

This patch workarounds this issue by checking the properties in RTCSession - the only place they are used. After this fix we managed to get a working call in FF 22.