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: https://v8.googlecode.com/svn/branches/bleeding_edge@23826 ce2b1a6d-e550-0410-aec6-3dcde31c8c00
Fix regress-411210 after r23824.

BUG=
R=hpayer@chromium.org

Review URL: https://codereview.chromium.org/559863004

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@23827 ce2b1a6d-e550-0410-aec6-3dcde31c8c00