Separate objects into two groups: reachable from a window (user),
and unreachable (system). Then do not take into account links
that come from system group to the user group when calculating
dominance relation.

Review URL: https://chromiumcodereview.appspot.com/10086006

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@11335 ce2b1a6d-e550-0410-aec6-3dcde31c8c00