Remove the HTMLMediaElementControllerNotNull use counter

The counter was added to help decide what to do with MediaController.
Now that MediaController has been unshipped, the use counter won't
provide useful data.

BUG=none

Review URL: https://codereview.chromium.org/260583002

git-svn-id: svn://svn.chromium.org/blink/trunk@172949 bbb929c8-8fbe-4397-9dbb-9b2b20218538