Bug 881978 - Make WebVTTLoadListener callbacks private. r=rillian

WebVTTLoadListener::OnParsedCue() and WebVTTLoadListener::OnReportError() are
for private use and should not be exposed publicly.
Bug 881517 - Just use Object.prototype for vanilla XPCWNs. r=mrbkap
Bug 881517 - Remove mPrototypeNoHelper machinery. r=mrbkap

The comments surrounding it don't make any sense to me, and most likely applied
to a pre-CPG world.