Introduce FullscreenElementStack::fullyExitFullscreen()

http://fullscreen.spec.whatwg.org/#fully-exit-fullscreen

BUG=383813

Review URL: https://codereview.chromium.org/387233002

git-svn-id: svn://svn.chromium.org/blink/trunk@178152 bbb929c8-8fbe-4397-9dbb-9b2b20218538