Fix d8-readline to use modern Persistent API

R=dcarney@chromium.org, svenpanne@chromium.org

Review URL: https://codereview.chromium.org/23537060

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@16870 ce2b1a6d-e550-0410-aec6-3dcde31c8c00