Set fullscreenEnabled to false when fullscreen is not supported.

According to the latest version of the living standard,
fullscreenEnabled must be false if fullscreen is not supported:
http://fullscreen.spec.whatwg.org/

More details about the latest update to the standard:
https://www.w3.org/Bugs/Public/show_bug.cgi?id=26479
https://github.com/whatwg/fullscreen/commit/d35a070d443632d0071b49318e40092667ac9801

Review URL: https://codereview.chromium.org/426593010

git-svn-id: svn://svn.chromium.org/blink/trunk@179520 bbb929c8-8fbe-4397-9dbb-9b2b20218538