Use heap iterator in store buffer when page was swept precisely.

src/objects.h

Index: src/objects.h
diff --git a/src/objects.h b/src/objects.h
index e32b51d2ff06cff2fb9a5fbb9f0704fdf094fb58..ce4aa24902343cda538fd4320027e6d0c60f799d 100644
--- a/src/objects.h
+++ b/src/objects.h
@@ -1747,6 +1747,10 @@ class HeapObject: public Object {
   // Returns the heap object's size in bytes
   inline int Size();
 
+  // Returns true if this heap object contains only references to other
+  // heap objects.
+  inline bool ContainsPointers();
+
   // Given a heap object's map pointer, returns the heap size in bytes
   // Useful when the map pointer field is used for other purposes.
   // GC internal.