This is a preparation for removing aggregated heap snapshots.
W/o this API, counting object instances in a snapshot is very hard.

R=sgjesse@chromium.org
BUG=1481
TEST=cctest/test-heap-profiler/NodesIteration

Review URL: http://codereview.chromium.org/7204040

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@8342 ce2b1a6d-e550-0410-aec6-3dcde31c8c00