Oilpan: Use transition types for VTTCue and VTTElement.

R=haraken@chromium.org
BUG=357163

Review URL: https://codereview.chromium.org/270313002

git-svn-id: svn://svn.chromium.org/blink/trunk@173526 bbb929c8-8fbe-4397-9dbb-9b2b20218538