Add @@iterator for generator objects

R=arv@chromium.org, rossberg@chromium.org
BUG=

Review URL: https://codereview.chromium.org/328093002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21797 ce2b1a6d-e550-0410-aec6-3dcde31c8c00