Make content settings UI classes not depend on the exact set of content types.

      This makes it a bit easier to add new content types that aren't exposed in the UI.

BUG=83972

Review URL: http://codereview.chromium.org/6992048

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@86875 0039d316-1c4b-4281-b951-d872f2087c98