Approximation is done by building a dominators tree for the heap graph.
Dominator nodes and retained sizes are serialized into JSON.

Removed:
 - reachable size (it is useless, after all);
 - HeapEntryCalculatedData (size is now stored in the node, retaining
   paths in a hash map);

Review URL: http://codereview.chromium.org/5154007

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@5867 ce2b1a6d-e550-0410-aec6-3dcde31c8c00