When tracing, we abuse the map for marking, thereby mutating it.
HeapObject::map() takes care of recovering unabused value.

R=yangguo@chromium.org

Review URL: https://codereview.chromium.org/316533002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21626 ce2b1a6d-e550-0410-aec6-3dcde31c8c00