scavenging of objects pointed to by weak handles earlier.  Rename
"mark" => "front" and "top" => "rear" to make it clearer which end is
which.
Review URL: http://codereview.chromium.org/113097

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@1900 ce2b1a6d-e550-0410-aec6-3dcde31c8c00