This unifies the iteration logic of objects groups iteration in global
handles. The scavenger as well as mark-and-compact now rely on the same
underlying logic.

R=ulan@chromium.org

Review URL: https://codereview.chromium.org/11299248

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@13128 ce2b1a6d-e550-0410-aec6-3dcde31c8c00