Oilpan: remove unnecessary TextTrackList finalization.

R=haraken@chromium.org
BUG=

Review URL: https://codereview.chromium.org/462653002

git-svn-id: svn://svn.chromium.org/blink/trunk@179954 bbb929c8-8fbe-4397-9dbb-9b2b20218538