Revert "Fix Heap::IsHeapIterable." (again)

This reverts commit r21397.

TBR=hpayer@chromium.org

Review URL: https://codereview.chromium.org/299813002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21404 ce2b1a6d-e550-0410-aec6-3dcde31c8c00