Bug fix: Memory leak of VTTCue

The current implementation doesn't release a VTTCue object at V8TextTrackCue::constructorCustom. This causes a memory leak at media/track/text-track-cue-constructor.html of the layout tests.

Review URL: https://codereview.chromium.org/99703005

git-svn-id: svn://svn.chromium.org/blink/trunk@163951 bbb929c8-8fbe-4397-9dbb-9b2b20218538