Revert "Fix Heap::IsHeapIterable."

This reverts commit r21387.

TBR=hpayer@chromium.org

Review URL: https://codereview.chromium.org/291193002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21388 ce2b1a6d-e550-0410-aec6-3dcde31c8c00