Add tests for removing the parent/child of the fullscreen element

The parent test would fail if Fullscreen::elementRemoved() were only
called for the top-most removed element.

The child test would require a more serious misinterpretation of "in
fullscreen element stack".

BUG=383813

Review URL: https://codereview.chromium.org/479113003

git-svn-id: svn://svn.chromium.org/blink/trunk@180700 bbb929c8-8fbe-4397-9dbb-9b2b20218538