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

base/process_util_mac.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 "base/process_util.h"
 
 #import <Cocoa/Cocoa.h>
 #include <crt_externs.h>
 #include <dlfcn.h>
 #include <errno.h>
@@ skipping 816 matching lines @@
         !g_old_memalign_purgeable) << "Old allocators unexpectedly non-null";
 
   // See http://trac.webkit.org/changeset/53362/trunk/Tools/DumpRenderTree/mac
   bool zone_allocators_protected = base::mac::IsOSLionOrLater();
 
   ChromeMallocZone* default_zone =
       reinterpret_cast<ChromeMallocZone*>(malloc_default_zone());
   ChromeMallocZone* purgeable_zone =
       reinterpret_cast<ChromeMallocZone*>(GetPurgeableZone());
 
-  vm_address_t page_start_default = NULL;
-  vm_address_t page_start_purgeable = NULL;
+  vm_address_t page_start_default = 0;
+  vm_address_t page_start_purgeable = 0;
   vm_size_t len_default = 0;
   vm_size_t len_purgeable = 0;
   if (zone_allocators_protected) {
     page_start_default = reinterpret_cast<vm_address_t>(default_zone) &
         static_cast<vm_size_t>(~(getpagesize() - 1));
     len_default = reinterpret_cast<vm_address_t>(default_zone) -
         page_start_default + sizeof(ChromeMallocZone);
     mprotect(reinterpret_cast<void*>(page_start_default), len_default,
              PROT_READ | PROT_WRITE);
 
@@ skipping 303 matching lines @@
   }
 }
 
 }  // namespace
 
 void EnsureProcessTerminated(ProcessHandle process) {
   WaitForChildToDie(process, kWaitBeforeKillSeconds);
 }
 
 }  // namespace base