Sync fullscreen element removal with the spec (again)

http://fullscreen.spec.whatwg.org/#model
https://www.w3.org/Bugs/Public/show_bug.cgi?id=26568#c12
https://www.w3.org/Bugs/Public/show_bug.cgi?id=26568#c16

BUG=383813
NOTRY=true

Review URL: https://codereview.chromium.org/477133002

git-svn-id: svn://svn.chromium.org/blink/trunk@180435 bbb929c8-8fbe-4397-9dbb-9b2b20218538