Bug 833385 - Hide TextTrack interfaces behind a preference.

Add a media.webvtt.enabled preference, defaulting to false.

Use the [Pref] webidl decorator to conditionalize the interfaces.

Check the pref in WebVTTLoadListener::LoadResource.

Check the pref in NS_NewHTMLTrackElement and return an
HTMLUnknownElement instead if it's disabled. This prevents
any of the attribute implementations from being called.