Protect against API abuse.

This makes Chrome's --single-process work again, but the real solution
is to pass an Isolate explicitly and fixing things on the Chrome side
=> follow-up CLs.

R=ulan@chromium.org

Review URL: https://codereview.chromium.org/239513006

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@20787 ce2b1a6d-e550-0410-aec6-3dcde31c8c00