Move objects-visiting into heap.

BUG=
R=jochen@chromium.org, mstarzinger@chromium.org

Review URL: https://codereview.chromium.org/443933002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@22971 ce2b1a6d-e550-0410-aec6-3dcde31c8c00