Add a UseCounter for legacy fullscreen key event filtering

Mozilla have already dropped the flags argument to mozRequestFullScreen,
which is where ALLOW_KEYBOARD_INPUT is specified.

Review URL: https://codereview.chromium.org/358283002

git-svn-id: svn://svn.chromium.org/blink/trunk@177198 bbb929c8-8fbe-4397-9dbb-9b2b20218538