Assert we have a parser handle before calling it.

And issue a warning if parsing fails.
Bug 833385 - Improve inheritance formatting.

This form allows subsequent commits to add classes
without touching the final line to add a comma.
Bug 852350 - Support [Pref] decorator on interfaces.

Add a new method to generate a direct check calling
Preferences::GetBool().

Work in progress checkpoint. Doesn't work.
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.