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

src/objects.h

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef V8_OBJECTS_H_
 #define V8_OBJECTS_H_
 
 #include <iosfwd>
 
 #include "src/allocation.h"
@@ skipping 9577 matching lines @@
                               kSize> BodyDescriptor;
 
  private:
   DECL_ACCESSORS(type_raw, Object)
   DISALLOW_IMPLICIT_CONSTRUCTORS(PropertyCell);
 };
 
 
 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)
 
   DECLARE_PRINTER(WeakCell)
   DECLARE_VERIFIER(WeakCell)
 
   // Layout description.
   static const int kValueOffset = HeapObject::kHeaderSize;
   static const int kNextOffset = kValueOffset + kPointerSize;
@@ skipping 1304 matching lines @@
     } else {
       value &= ~(1 << bit_position);
     }
     return value;
   }
 };
 
 } }  // namespace v8::internal
 
 #endif  // V8_OBJECTS_H_