Bug 879669 - Part 1: Don't callback if there is no track in MediaStream. r=roc
Bug 879669 - Part 2: Let Encoder can support various mimeType. r=roc
Bug 879669 - Part 3: Start recording when tracks have been added. r=roc
Bug 879669 - Part 4: Stop can cancel the initialization of start method, change to check the blob case by case. r=roc, r=jsmith