Bug 1053130: Refactor MediaEngine video sources; alse remove Snapshot function. r=jesup,alfredo

--HG--
rename : content/media/webrtc/MediaEngineWebRTCVideo.cpp => content/media/webrtc/MediaEngineCameraVideoSource.cpp
rename : content/media/webrtc/MediaEngineWebRTC.h => content/media/webrtc/MediaEngineCameraVideoSource.h
rename : content/media/webrtc/MediaEngineWebRTCVideo.cpp => content/media/webrtc/MediaEngineGonkVideoSource.cpp
rename : content/media/webrtc/MediaEngineWebRTC.h => content/media/webrtc/MediaEngineGonkVideoSource.h