Bug 891705 - [MediaEncoder] Implement WebM 1.0 container writer. r=giles, r=gps, r=rillian

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
Bug 957439 - Do not assert at Pause or Resume on unsupported input stream. r=roc, r=jsmith