Use WeakPtr to refer to VTTCue in VTTCueBox

Use WeakPtr to refer to VTTCue in VTTCueBox
https://bugs.webkit.org/show_bug.cgi?id=192575

Reviewed by Eric Carlson.

Address the FIXME in VTTCue::~VTTCue by clearing VTTCueBox::m_cue when VTTCue goes away.
This is implemented by simply using WeakPtr.

No new tests since there shoul be no behaivoral change.

* html/track/TextTrackCueGeneric.cpp:
(WebCore::TextTrackCueGenericBoxElement::applyCSSProperties):
* html/track/VTTCue.cpp:
(WebCore::VTTCueBox::VTTCueBox):
(WebCore::VTTCueBox::getCue const):
(WebCore::VTTCueBox::applyCSSProperties):
(WebCore::VTTCue::~VTTCue):
* html/track/VTTCue.h:
(WebCore::VTTCueBox::fontSizeFromCaptionUserPrefs const):

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@239063 268f45cc-cd09-0410-ab3c-d52691b4dbfc