Hide the CC button for <audio> with <track>s

It is per spec that the DOM API should work the same for <audio>
and <video>, but it doesn't make any sense to have the UI bits for
<audio> since there's nowhere to actually render the cues.

BUG=314027

Review URL: https://codereview.chromium.org/54713007

git-svn-id: svn://svn.chromium.org/blink/trunk@161246 bbb929c8-8fbe-4397-9dbb-9b2b20218538