855100 - Initialize HTMLTrackElement::mTrack in the constructor.

Add a new default TextTrack constructor and use it to initialize
the HTMLTrackElement's mTrack member.

Previously the constructor just set it to null, but the bindings
don't handle that case, and we would segfault accessing the
'track' property on a track element created outside the parser:

  document.createElement('track').track;

The initial element is created disabled, since it has nothing to
play. Thanks to Marcus Saad for help with the patch.