Sync fullscreen element removal with the spec

http://fullscreen.spec.whatwg.org/#model
https://www.w3.org/Bugs/Public/show_bug.cgi?id=26326

TEST=LayoutTests/fullscreen/model/remove-*.html (new)
TEST=LayoutTests/fullscreen/full-screen-remove-*.html (no regression)

The distinction between fullyExitFullscreen() and exitFullscreen() was
not tested, but the new remove-last.html test fails with the old code.

BUG=383813

Review URL: https://codereview.chromium.org/476433002

git-svn-id: svn://svn.chromium.org/blink/trunk@180259 bbb929c8-8fbe-4397-9dbb-9b2b20218538