Implement HeapIterator that skips over unreachable objects.

Implement HeapIterator that skips over unreachable objects.

I'm using it when creating heap snapshots. I decided that it will
be more convenient to have it as a separate piece of code, instead
of embedding into the snapshot generator.

Review URL: http://codereview.chromium.org/6014004

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@6091 ce2b1a6d-e550-0410-aec6-3dcde31c8c00