Provide more accurate results about used heap size via GetHeapStatistics.

src/heap.cc

Index: src/heap.cc
diff --git a/src/heap.cc b/src/heap.cc
index 134f40e5070cf80d41978653d7406120d28a58fa..8cc6d7ee2409f71099fba04d4d2644091c078bcf 100644
--- a/src/heap.cc
+++ b/src/heap.cc
@@ -400,6 +400,17 @@ intptr_t Heap::SizeOfObjects() {
   return total;
 }
 
+intptr_t Heap::SizeOfObjectsSlow() {
+  intptr_t total = 0;
+  HeapIterator iterator;
+  for (HeapObject* obj = iterator.next(); obj != NULL; obj = iterator.next()) {
+    if (!FreeListNode::IsFreeListNode(obj)) {
+      total += obj->Size();
+    }
+  }
+  return total;
+}
+
 void Heap::GarbageCollectionEpilogue() {
 #ifdef DEBUG
   allow_allocation(true);