Unprefix most methods on FullscreenElementStack

fullscreenEnabled no longer even considers the webkitallowfullscreen
attribute, and the prefixes do no good internally anyway. They are
left only on the Mozilla-style API since they should generally not be
used other than for bindings.

It turned out that the Internals wrappers were unused. They were added
in http://trac.webkit.org/changeset/114790 "to facilitate writing
reproducible LayoutTests", but no tests using them were ever added.

BUG=383813

Review URL: https://codereview.chromium.org/390363002

git-svn-id: svn://svn.chromium.org/blink/trunk@178339 bbb929c8-8fbe-4397-9dbb-9b2b20218538