Bug 868509 - Rename TextTrackCue to VTTCue r=rillian

This way we can ensure that JS code written for WebVTT is spec
compliant while at the same time not having to abstract VTTCue
from TextTrackCue.

--HG--
rename : dom/webidl/TextTrackCue.webidl => dom/webidl/VTTCue.webidl