Allow externalizing cons and sliced string.
Allow externalizing cons and sliced string.

Fulfill overdue TODO.

R=mstarzinger@chromium.org

Committed: https://code.google.com/p/v8/source/detail?r=23058

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

git-svn-id: f98b9d40cb02301bc76fa9e1b46ee668158567fd@23826 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

UniversalFuture/moosh,UniversalFuture/moosh,UniversalFuture/moosh,UniversalFuture/moosh