Stop using controller as a variable name for FullscreenElementStack

This was probably originally FullscreenController.

Also fix a typo and simplify isActiveFullScreenElement.

Review URL: https://codereview.chromium.org/421693003

git-svn-id: svn://svn.chromium.org/blink/trunk@179009 bbb929c8-8fbe-4397-9dbb-9b2b20218538