Convert GN visibility to be lists.

This is a followup to my previous patch that missed this case.

R=kjellander@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/24529004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@7137 4adac7df-926f-26a2-2b94-8c16560cd09d