Fix bogus assert in HeapIterator.

BUG=397485
LOG=n
R=jarin@chromium.org

Review URL: https://codereview.chromium.org/416403011

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@22642 ce2b1a6d-e550-0410-aec6-3dcde31c8c00