As taking a snapshot of a large heap takes noticeable time, it's
good to be able to monitor and control it.

The change itself is small, big code deletes and additions are in
fact moves. The only significant change is simplification of
approximated retained sizes calculation algorithm.

Review URL: http://codereview.chromium.org/5687003

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@5978 ce2b1a6d-e550-0410-aec6-3dcde31c8c00