Initialize pre-allocated fields of JSObject with undefined.

src/objects.h

 // Copyright 2011 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 1912 matching lines @@
   inline Object* FastPropertyAtPut(int index, Object* value);
 
   // Access to in object properties.
   inline int GetInObjectPropertyOffset(int index);
   inline Object* InObjectPropertyAt(int index);
   inline Object* InObjectPropertyAtPut(int index,
                                        Object* value,
                                        WriteBarrierMode mode
                                        = UPDATE_WRITE_BARRIER);
 
-  // initializes the body after properties slot, properties slot is
-  // initialized by set_properties
+  // Initializes the body after properties slot, properties slot is
+  // initialized by set_properties.  Fill the pre-allocated fields with
+  // pre_allocated_value and the rest with filler_value.
   // Note: this call does not update write barrier, it is caller's
   // reponsibility to ensure that *v* can be collected without WB here.
-  inline void InitializeBody(int object_size, Object* value);
+  inline void InitializeBody(Map* map,
+                             Object* pre_allocated_value,
+                             Object* filler_value);
 
   // Check whether this object references another object
   bool ReferencesObject(Object* obj);
 
   // Casting.
   static inline JSObject* cast(Object* obj);
 
   // Disalow further properties to be added to the object.
   MUST_USE_RESULT MaybeObject* PreventExtensions();
 
@@ skipping 5522 matching lines @@
     } else {
       value &= ~(1 << bit_position);
     }
     return value;
   }
 };
 
 } }  // namespace v8::internal
 
 #endif  // V8_OBJECTS_H_