Disable cloud policy for extensions by default.

The flag will be flipped back once the server is ready. For now it'll be
disabled on cloud policy. Note that policy for extensions is still loaded from
the platform policy providers.

BUG=108992

Review URL: https://codereview.chromium.org/107063002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@239198 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 239198