Make text track and cue wrapper objects dependent on their media element.

If a TextTrack or VTTCue (TextTrackCue) object is added to a media
element, indirectly by way of addCue() or addTextTrack(), arrange for
their wrapper objects to be registered as dependents  of the media
element's wrapper so as to keep these wrapper objects alive.

This is done so that when we later need to return back a wrapper
for the underlying track/cue object, the same wrapper object is returned.

R=acolwell@chromium.org
BUG=358109

Review URL: https://codereview.chromium.org/242113012

git-svn-id: svn://svn.chromium.org/blink/trunk@172065 bbb929c8-8fbe-4397-9dbb-9b2b20218538