Allow externalizing cons and sliced string.

Fulfill overdue TODO.

R=mstarzinger@chromium.org

Review URL: https://codereview.chromium.org/460643002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@23058 ce2b1a6d-e550-0410-aec6-3dcde31c8c00