Remove the disallowFullscreenForNonMediaElements setting.

This setting will not longer be used after:
https://codereview.chromium.org/618013003/

It will be landed after that change.

BUG=398485

Review URL: https://codereview.chromium.org/614173003

git-svn-id: svn://svn.chromium.org/blink/trunk@183900 bbb929c8-8fbe-4397-9dbb-9b2b20218538