Note that we currently have no such transitions, but we will in the future, and
we have to avoid keeping maps live too long.

Review URL: https://chromiumcodereview.appspot.com/9212045

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@10495 ce2b1a6d-e550-0410-aec6-3dcde31c8c00