Revert 167021, info lead to bug-fix.

chrome/common/mac/objc_zombie.mm

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #import "chrome/common/mac/objc_zombie.h"
 
 #include <AvailabilityMacros.h>
 
 #include <execinfo.h>
 #import <objc/runtime.h>
@@ skipping 98 matching lines @@
   Class wasa = object_getClass(self);
   const size_t size = class_getInstanceSize(wasa);
 
   // Destroy the instance by calling C++ destructors and clearing it
   // to something unlikely to work well if someone references it.
   // NOTE(shess): |object_dispose()| will call this again when the
   // zombie falls off the treadmill!  But by then |isa| will be a
   // class without C++ destructors or associative references, so it
   // won't hurt anything.
   objc_destructInstance(self);
-  // TODO(shess): Temporarily disable clearing the object to debug
-  // http://crbug.com/154483
-#if 0
   memset(self, '!', size);
-#endif
 
   // If the instance is big enough, make it into a fat zombie and have
   // it remember the old |isa|.  Otherwise make it a regular zombie.
   // Setting |isa| rather than using |object_setClass()| because that
   // function is implemented with a memory barrier.  The runtime's
   // |_internal_object_dispose()| (in objc-class.m) does this, so it
   // should be safe (messaging free'd objects shouldn't be expected to
   // be thread-safe in the first place).
 #pragma clang diagnostic push  // clang warns about direct access to isa.
 #pragma clang diagnostic ignored "-Wdeprecated-objc-isa-usage"
@@ skipping 298 matching lines @@
   if (oldZombies) {
     for (size_t i = 0; i < oldCount; ++i) {
       if (oldZombies[i].object)
         object_dispose(oldZombies[i].object);
     }
     free(oldZombies);
   }
 }
 
 }  // namespace ObjcEvilDoers