Rename FullscreenElementStack to just Fullscreen

This class does more than just manage the fullscreen element stack, and
the shorter name makes the code more readable in many places. In the
renamed Fullscreen class, it also makes it more obvious when the actual
fullscreen element stack is being manipulated.

BUG=383813

Review URL: https://codereview.chromium.org/476763004

git-svn-id: svn://svn.chromium.org/blink/trunk@180433 bbb929c8-8fbe-4397-9dbb-9b2b20218538