objects retained by object groups and global handles.

This information is then used during heap snapshot generation
to provide a more complete memory picture.

This patch will be needed to fix https://bugs.webkit.org/show_bug.cgi?id=53659.

Review URL: http://codereview.chromium.org/6626043

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@7125 ce2b1a6d-e550-0410-aec6-3dcde31c8c00