Disable externalization of sliced/cons strings in old pointer space.

This is a temporary workaround for the fact that a good deal of the code that triggers access checks, which in turn externalize strings, is not yet handlified and therefore not GC safe.

R=yangguo@chromium.org

Review URL: https://codereview.chromium.org/25146008

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@17023 ce2b1a6d-e550-0410-aec6-3dcde31c8c00