Turn old space cons strings into regular external strings (not short).

R=hpayer@chromium.org
BUG=v8:3530
LOG=N

Review URL: https://codereview.chromium.org/368223002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@23794 ce2b1a6d-e550-0410-aec6-3dcde31c8c00