Remove FullScreen from RuntimeEnabledFeatures

Chromium no longer has a commandline to disable/enable FullScreen.
http://code.google.com/p/chromium/issues/detail?can=2&q=343996
https://groups.google.com/a/chromium.org/forum/#!topic/chromium-dev/nEbzlCu0hto

BUG=344002

Review URL: https://codereview.chromium.org/187763003

git-svn-id: svn://svn.chromium.org/blink/trunk@168714 bbb929c8-8fbe-4397-9dbb-9b2b20218538