This is intended for smoother migration to the new API in Chromium.
Also, aggregated heap snapshots can be used for cheaply obtaining
heap statistics, e.g. in tests.

Review URL: http://codereview.chromium.org/3124024

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@5297 ce2b1a6d-e550-0410-aec6-3dcde31c8c00