Gecko:MediaRecorder

Reference spec:
https://dvcs.w3.org/hg/dap/raw-file/tip/media-stream-capture/RecordingProposal.html
bugs:
bug 803414 Audio Recording - Web API & Implementation
bug 834165 Implement BlobEvent
bug 825110 [meta] Porting WebRTC video_module for B2G
bug 825112 [meta] Porting WebRTC audio_module for B2G
media encoder
audio encoder
video encoder
media writer
ogg writer