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>
 
 #include "base/basictypes.h"
 #include "base/compiler_specific.h"
 
 namespace base {
 namespace mac {
 
 // Just like ScopedCFTypeRef but for io_object_t and subclasses.
 template<typename IOT>
 class ScopedIOObject {
  public:
   typedef IOT element_type;
 
-  explicit ScopedIOObject(IOT object = NULL)
+  explicit ScopedIOObject(IOT object = IO_OBJECT_NULL)
       : object_(object) {
   }
 
   ~ScopedIOObject() {
     if (object_)
       IOObjectRelease(object_);
   }
 
-  void reset(IOT object = NULL) {
+  void reset(IOT object = IO_OBJECT_NULL) {
     if (object_)
       IOObjectRelease(object_);
     object_ = object;
   }
 
   bool operator==(IOT that) const {
     return object_ == that;
   }
 
   bool operator!=(IOT that) const {
     return object_ != that;
   }
 
   operator IOT() const {
     return object_;
   }
 
   IOT get() const {
     return object_;
   }
 
   void swap(ScopedIOObject& that) {
     IOT temp = that.object_;
     that.object_ = object_;
     object_ = temp;
   }
 
   IOT release() WARN_UNUSED_RESULT {
     IOT temp = object_;
-    object_ = NULL;
+    object_ = IO_OBJECT_NULL;
     return temp;
   }
 
  private:
   IOT object_;
 
   DISALLOW_COPY_AND_ASSIGN(ScopedIOObject);
 };
 
 }  // namespace mac
 }  // namespace base
 
 #endif  // BASE_MAC_SCOPED_IOOBJECT_H_