Add option to compute average scavenge speed w.r.t survived objects.

src/heap/heap.h

Index: src/heap/heap.h
diff --git a/src/heap/heap.h b/src/heap/heap.h
index dd6a571eaf3ad77216e2c710c3a1a904f4b24f80..db57591984d341f0c893b603deb2e760a63b9c73 100644
--- a/src/heap/heap.h
+++ b/src/heap/heap.h
@@ -1277,6 +1277,10 @@ class Heap {
     semi_space_copied_object_size_ += object_size;
   }
 
+  inline intptr_t SurvivedNewSpaceObjectSize() {
+    return promoted_objects_size_ + semi_space_copied_object_size_;
+  }
+
   inline void IncrementNodesDiedInNewSpace() { nodes_died_in_new_space_++; }
 
   inline void IncrementNodesCopiedInNewSpace() { nodes_copied_in_new_space_++; }