Use the VTTCue constructor instead of TextTrackCue in test

The TextTrackCue constructor is being deprecated.

BUG=334942

Review URL: https://codereview.chromium.org/140563002

git-svn-id: svn://svn.chromium.org/blink/trunk@165199 bbb929c8-8fbe-4397-9dbb-9b2b20218538