Ignore MediaController in the closed captions button logic

Involving the media controller plainly does not make sense.

There was no test coverage for the CC button with a media controller
present, and adding a negative test for it does not seem useful.

Note that what remains is still not great since it doesn't allow you to
pick between multiple tracks, but that will be fixed at a later time.

BUG=341813
TEST=LayoutTests/media/audio-controls-captions.html
     LayoutTests/media/video-controls-captions.html

Review URL: https://codereview.chromium.org/201123004

git-svn-id: svn://svn.chromium.org/blink/trunk@169405 bbb929c8-8fbe-4397-9dbb-9b2b20218538