Added new GetHeapStatistics API entry and deprecated old one.

Simplified the HeapStatistics class a bit, following Uncle Bob's advice that
adding accessors to DTOs only satisfies some design fundamentalists, but serves
no other purpose. :-)

Review URL: https://codereview.chromium.org/12207076

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@13631 ce2b1a6d-e550-0410-aec6-3dcde31c8c00