Make sure media/mtransport/nr_socket_prsock.cpp is disabled in Tor Browser
Currently we don't compile media/mtransport/nr_socket_prsock.cpp in as we use
--disable-webrtc. There is the idea to move to a preference-based approach for disabling webrtc, though (https://bugzilla.mozilla.org/show_bug.cgi?id=1314443). If that is done we need to be sure that media/mtransport/nr_socket_prsock.cpp stays unavailable, though.