Use smi zero instead of undefine_value to zap dead weak cells.

src/objects.h

Index: src/objects.h
diff --git a/src/objects.h b/src/objects.h
index 603f7b3e3f8a78a2c4be4178cb9d34f33321ccf9..1f44187ccb21f08e55e64d411035ff9459a7377a 100644
--- a/src/objects.h
+++ b/src/objects.h
@@ -9595,14 +9595,16 @@ class PropertyCell: public Cell {
 
 class WeakCell : public HeapObject {
  public:
-  inline HeapObject* value() const;
+  inline Object* value() const;
 
   // This should not be called by anyone except GC.
-  inline void clear(HeapObject* undefined);
+  inline void clear();
 
   // This should not be called by anyone except allocator.
   inline void initialize(HeapObject* value);
 
+  inline bool cleared() const;
+
   DECL_ACCESSORS(next, Object)
 
   DECLARE_CAST(WeakCell)