H.264 using Olga's FFmpeg framework

[classilla]

Starting point:
https://github.com/OlgaTPark/tenfourfox/commit/a22d83f6b190a71386fe193ee1f3f08dc1639d39.diff

[classilla]

See also OlgaTPark#1

[classilla]

Need to pref on media.mediasource.mp4.enabled as well.

[classilla]

shippin'