Use system constants instead of NULL for unset PID.

base/process.h

Index: base/process.h
===================================================================
--- base/process.h	(revision 80123)
+++ base/process.h	(working copy)
@@ -25,11 +25,13 @@
 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)