Ensure that collection iterators' maps have constructors

This fixes JSObject::GetCreationContext() for such iterators.

R=rossberg@chromium.org

Review URL: https://codereview.chromium.org/288233002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21333 ce2b1a6d-e550-0410-aec6-3dcde31c8c00