Use system constants instead of NULL for unset PID.

base/process.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_PROCESS_H_
 #define BASE_PROCESS_H_
 #pragma once
 
 #include "base/base_api.h"
 #include "base/basictypes.h"
 #include "build/build_config.h"
 
 #include <sys/types.h>
 #if defined(OS_WIN)
 #include <windows.h>
 #endif
 
 namespace base {
 
 // ProcessHandle is a platform specific type which represents the underlying OS
 // handle to a process.
 // ProcessId is a number which identifies the process in the OS.
 #if defined(OS_WIN)
 typedef HANDLE ProcessHandle;
 typedef DWORD ProcessId;
 typedef HANDLE UserTokenHandle;
 const ProcessHandle kNullProcessHandle = NULL;
+const ProcessId kNullProcessId = NULL;

[amit, 2011/04/01 05:15:12]

Since PricessId is a DWORD here, why not = 0? 

[Jeff Bailey (chromium), 2011/04/01 05:26:09]

Had to look up what a DWORD was.  Will change is.  I had put it as NULL before
just to match the previous definition, but this looks more correct.

 #elif defined(OS_POSIX)
 // On POSIX, our ProcessHandle will just be the PID.
 typedef pid_t ProcessHandle;
 typedef pid_t ProcessId;
 const ProcessHandle kNullProcessHandle = 0;
+const ProcessId kNullProcessId = 0;
 #endif  // defined(OS_WIN)
 
 #if defined(OS_POSIX) && !defined(OS_MACOSX)
 // saved_priority_ will be set to this to indicate that it's not holding
 // a valid value. -20 to 19 are valid process priorities.
 const int kUnsetProcessPriority = 256;
 #endif
 
 class BASE_API Process {
  public:
@@ skipping 56 matching lines @@
 #if defined(OS_POSIX) && !defined(OS_MACOSX)
   // Holds the priority that the process was set to when it was backgrounded.
   // If the process wasn't backgrounded it will be kUnsetProcessPriority.
   int saved_priority_;
 #endif
 };
 
 }  // namespace base
 
 #endif  // BASE_PROCESS_H_