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

chrome/browser/mac/install_from_dmg.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.
 
 #include "chrome/browser/mac/install_from_dmg.h"
 
 #include <ApplicationServices/ApplicationServices.h>
 #import <AppKit/AppKit.h>
 #include <CoreFoundation/CoreFoundation.h>
 #include <CoreServices/CoreServices.h>
@@ skipping 64 matching lines @@
 
   io_iterator_t iterator_ref;
   kern_return_t kr =
       IORegistryEntryCreateIterator(media,
                                     kIOServicePlane,
                                     kIORegistryIterateRecursively |
                                         kIORegistryIterateParents,
                                     &iterator_ref);
   if (kr != KERN_SUCCESS) {
     LOG(ERROR) << "IORegistryEntryCreateIterator: " << kr;
-    return NULL;
+    return IO_OBJECT_NULL;
   }
   base::mac::ScopedIOObject<io_iterator_t> iterator(iterator_ref);
-  iterator_ref = NULL;
+  iterator_ref = IO_OBJECT_NULL;
 
   // Look at each of the ancestor services, beginning with the parent,
   // iterating all the way up to the device tree's root. If any ancestor
   // service matches the class used for disk images, the media resides on a
   // disk image, and the disk image file's path can be determined by examining
   // the image-path property.
   for (base::mac::ScopedIOObject<io_service_t> ancestor(
            IOIteratorNext(iterator));
        ancestor;
        ancestor.reset(IOIteratorNext(iterator))) {
     if (IOObjectConformsTo(ancestor, disk_image_class)) {
       return ancestor.release();
     }
   }
 
   // The media does not reside on a disk image.
-  return NULL;
+  return IO_OBJECT_NULL;
 }
 
 // Given an io_service_t (expected to be of class IOMedia), determines whether
 // that service is on a disk image. If it is, returns true. If image_path is
 // present, it will be set to the pathname of the disk image file, encoded in
 // filesystem encoding.
 bool MediaResidesOnDiskImage(io_service_t media, std::string* image_path) {
   if (image_path) {
     image_path->clear();
   }
@@ skipping 90 matching lines @@
 
   io_iterator_t iterator_ref;
   kern_return_t kr = IOServiceGetMatchingServices(master_port,
                                                   match_dict,
                                                   &iterator_ref);
   if (kr != KERN_SUCCESS) {
     LOG(ERROR) << "IOServiceGetMatchingServices: " << kr;
     return false;
   }
   base::mac::ScopedIOObject<io_iterator_t> iterator(iterator_ref);
-  iterator_ref = NULL;
+  iterator_ref = IO_OBJECT_NULL;
 
   // There needs to be exactly one matching service.
   base::mac::ScopedIOObject<io_service_t> media(IOIteratorNext(iterator));
   if (!media) {
     LOG(ERROR) << "IOIteratorNext: no service";
     return false;
   }
   base::mac::ScopedIOObject<io_service_t> unexpected_service(
       IOIteratorNext(iterator));
   if (unexpected_service) {
@@ skipping 452 matching lines @@
   const UInt8* trash_path_u8 = reinterpret_cast<const UInt8*>(
       trash_path.value().c_str());
   status = FNNotifyByPath(trash_path_u8,
                           kFNDirectoryModifiedMessage,
                           kNilOptions);
   if (status != noErr) {
     OSSTATUS_LOG(ERROR, status) << "FNNotifyByPath";
     return;
   }
 }