chrome: return empty array from getSenders
chrome: return empty array from getSenders

makes getSenders return an empty array when called before addStream/addTrack.

No test. This belongs into a wpt test imo :-)

webrtc/adapter,jan-ivar/adapter,Acidburn0zzz/adapter,jan-ivar/adapter,webrtc/adapter,Acidburn0zzz/adapter,xdumaine/adapter,Acidburn0zzz/adapter,xdumaine/adapter