Bug 833385 - More cycle collection cleanup.
Use IsWebVTTEnabled in NS_NewHTMLTrackElement.

Rather than duplicating the PrefEnabled check on the binding.
Review request from bz.
Bug 833385 - Remove unused mCurrentLoadID.
Bug 833385 - Initialize HTMLTrackElement::mReadState.

Add an enum for the numeric states from the spec.
Postpone SetAcceptHeader and LoadReasource to bug 833382.