Provide more accurate results about used heap size via GetHeapStatistics.

Provide more accurate results about used heap size via GetHeapStatistics.

I observed that used heap size provided by Heap::SizeOfObjects() is
usually about ~10% bigger than the number calculated by summing up
heap objects sizes.

This aligns DevTools Timeline stats with Heap profiler stats.

Review URL: http://codereview.chromium.org/4888001

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@5825 ce2b1a6d-e550-0410-aec6-3dcde31c8c00