Make Zone::New() and Zone::NewArray() usable w/o v8.h.

R=svenpanne@chromium.org

Review URL: https://codereview.chromium.org/456663002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@22988 ce2b1a6d-e550-0410-aec6-3dcde31c8c00