Fix a few warnings that -Wnull-conversion of a future clang will complain about.

base/mac/scoped_ioobject.h

 // Copyright (c) 2011 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.
 
 #ifndef BASE_MAC_SCOPED_IOOBJECT_H_
 #define BASE_MAC_SCOPED_IOOBJECT_H_
 #pragma once
 
 #include <IOKit/IOKitLib.h>
 
@@ skipping 41 matching lines @@
   }
 
   void swap(ScopedIOObject& that) {
     IOT temp = that.object_;
     that.object_ = object_;
     object_ = temp;
   }
 
   IOT release() WARN_UNUSED_RESULT {
     IOT temp = object_;
-    object_ = NULL;
+    object_ = 0;

[Mark Mentovai, 2012/03/27 21:09:59]

It would be proper to use IO_OBJECT_NULL here.

     return temp;
   }
 
  private:
   IOT object_;
 
   DISALLOW_COPY_AND_ASSIGN(ScopedIOObject);
 };
 
 }  // namespace mac
 }  // namespace base
 
 #endif  // BASE_MAC_SCOPED_IOOBJECT_H_