Use hash map to look for objects in the root array when serializing.

R=mvstanton@chromium.org

Review URL: https://codereview.chromium.org/645533003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24826 ce2b1a6d-e550-0410-aec6-3dcde31c8c00