Move Web-facing fullscreen APIs to DocumentFullscreen and ElementFullscreen

This is closer to the partial interfaces of the spec:
http://fullscreen.spec.whatwg.org/#api

As a side-effect, the prefixed functions are now only used for bindings,
forcing internal clients to pick a request type more explicitly.

BUG=383813

Review URL: https://codereview.chromium.org/449133003

git-svn-id: svn://svn.chromium.org/blink/trunk@179805 bbb929c8-8fbe-4397-9dbb-9b2b20218538