Always invoke HeapObjectIterator::has_next() before invoking HeapObjectIterator::next().
This is necessary as ::has_next() has an important side-effect of going to the next
page when current page is exhausted.

And to find if pointers are encodable use more precise data---top of map space, not a number
of pages, as pages might stay in map space due to chunking.

Review URL: http://codereview.chromium.org/552066

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@3672 ce2b1a6d-e550-0410-aec6-3dcde31c8c00