firefox: unsafe RTCSessionDescription shim

[fippo]

from stackoverflow

module.exports = typeof RTCSessionDescription !== 'undefined'
  ? RTCSessionDescription
: mozRTCSessionDescription;

can go wrong if media.peerconnection.enabled is set to false in about:firefox.
The minimal modification here is probably to say : window.mozRTCSessionDescription

[graue]

There's a similar problem with a reference to RTCPeerConnection