Media API for ServoView

[ferjm]

This is the equivalent to https://bugzilla.mozilla.org/show_bug.cgi?id=1498246 for ServoView. From that issue:

Some context: On Firefox Reality we need a way to control playing media in addition to know some media events and metadata. This is required because a lot of 360/180 video websites (or just a raw video) do not have a WebVR player. Browsers are getting around this by allowing the user to map the video onto a sphere or half sphere and while the video is playing the controls are warped as well. So to allow the user to still control the video, they are given an external controller with play/pause/stop/volume setter and a playback seek bar. Standalone Oculus, Chrome and Samsung browsers have this capability.

[ferjm]

This would allow 360 video for Firefox Reality and it is independent of #21995 or #21996