Merge remote-tracking branch 'mozilla/master'

Conflicts:
	media/webrtc/signaling/src/media-conduit/VideoConduit.cpp
	media/webrtc/signaling/src/peerconnection/PeerConnectionCtx.cpp
	media/webrtc/signaling/src/peerconnection/PeerConnectionImpl.cpp
	media/webrtc/trunk/webrtc/modules/rtp_rtcp/source/rtp_receiver_video.cc
	modules/libpref/src/Preferences.cpp
	modules/libpref/src/init/all.js
	modules/libpref/src/prefapi.cpp
[Hack] Don't assert off main thread pref access
Re-add h261 related preferences
Fix detection of H261 in VideoConduit
Add possibility to cap H261 encoder framerate by pref
Cap H261 encoder framerate to 10 fps by default