Let FullscreenElementStack methods take references where possible

The main motivation is requestFullScreenForElement, which handled the
Element* argument being null even though there's no way that could
happen. The fallback to document()->documentElement() was unreachable.

Get rid of fullScreenIsAllowedForElement since it was only used in a
single place.

Review URL: https://codereview.chromium.org/359453002

git-svn-id: svn://svn.chromium.org/blink/trunk@176968 bbb929c8-8fbe-4397-9dbb-9b2b20218538