Sync "fully exit fullscreen" with the spec

http://fullscreen.spec.whatwg.org/#fully-exit-fullscreen
https://www.w3.org/Bugs/Public/show_bug.cgi?id=26587

This should not change observable behavior.

When to actually invoke "fully exit fullscreen" is not finalized:
https://www.w3.org/Bugs/Public/show_bug.cgi?id=26584

TEST=LayoutTests/fullscreen/full-screen-exit-when-popup.html (old)
TEST=LayoutTests/fullscreen/model/fully-exit-fullscreen-*.html (new)

The old test enters the fullyExitFullscreen() code path, but passes even
if fullyExitFullscreen() is made an alias of exitFullscreen().

The new tests will fail if a non-top-level document is used or if more
than one element is on the stack when exitFullscreen() is called.

BUG=403763

Review URL: https://codereview.chromium.org/470723002

git-svn-id: svn://svn.chromium.org/blink/trunk@180506 bbb929c8-8fbe-4397-9dbb-9b2b20218538