Make Fullscreen::fullyExitFullscreen() static

The implementation may not need the Fullscreen instance for the Document
fullyExitFullscreen() is given, and can avoid creating one for the
top-level Document in case there is no fullscreen element, which is >99%
of time.

BUG=403763

Review URL: https://codereview.chromium.org/485953003

git-svn-id: svn://svn.chromium.org/blink/trunk@180563 bbb929c8-8fbe-4397-9dbb-9b2b20218538