Fix assertion failure caused by external strings.

This fixes two issues:
- Update externalize-string-extension to the behavior of the API (see r18285)
- Convert cons strings in old pointer space to short external strings as
  expected by Heap::AllowedToBeMigrated, regardless of alignment.

R=mstarzinger@chromium.org

Review URL: https://codereview.chromium.org/189663011

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@19755 ce2b1a6d-e550-0410-aec6-3dcde31c8c00