This is an integrated patch which includes: 1. Bug 891705: [MediaEncoder] Implement WebM 1.0 container writer. r=giles, r=gps 2. Bug 950567: [MediaEncoder] Phase-in libmkv library. r=giles 3. bug 883749: Implement Vorbis encoding. r=rillian 4. bug 881840: Implement VP8 track encoder. r=rillian