Reland "Fix Heap::IsHeapIterable."

This relands r21388 (+ handlification of an offending function).

BUG=373283
LOG=N
R=hpayer@chromium.org

Review URL: https://codereview.chromium.org/294903003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21397 ce2b1a6d-e550-0410-aec6-3dcde31c8c00