Use portable typedef for PIDs (process IDs).

base/process.h

Index: base/process.h
diff --git a/base/process.h b/base/process.h
index de269397522cea0339b0c06a62ced37a5c0ad99f..312f84cacf855e6ef02abe3c61904684c9c2fd19 100644
--- a/base/process.h
+++ b/base/process.h
@@ -7,6 +7,7 @@
 
 #include "base/basictypes.h"
 
+#include <sys/types.h>
 #ifdef OS_WIN
 #include <windows.h>
 #endif
@@ -15,11 +16,14 @@ 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;
 #elif defined(OS_POSIX)
 // On POSIX, our ProcessHandle will just be the PID.
-typedef int ProcessHandle;
+typedef pid_t ProcessHandle;
+typedef pid_t ProcessId;
 #endif
 
 class Process {
@@ -37,7 +41,7 @@ class Process {
   void set_handle(ProcessHandle handle) { process_ = handle; }
 
   // Get the PID for this process.
-  int32 pid() const;
+  ProcessId pid() const;
 
   // Is the this process the current process.
   bool is_current() const;