Add UseCounter for TextTrackCue constructor

The plan in issue 270340 is to leave the TextTrackCue constructor until
it is known to be safe to deprecate and remove. Add a UseCounter so that
we can get stats from M32 stable, to be branched Nov 4th, 2013.

BUG=270340

Review URL: https://codereview.chromium.org/47453002

git-svn-id: svn://svn.chromium.org/blink/trunk@160707 bbb929c8-8fbe-4397-9dbb-9b2b20218538