Use portable typedef for PIDs (process IDs).

base/process_posix.cc

Index: base/process_posix.cc
diff --git a/base/process_posix.cc b/base/process_posix.cc
index f5a78219e81ee01487a45f9a894e9390dba1e530..ff8cf92c52e79ebf1534e85b77b9f4ad4a4e1df8 100644
--- a/base/process_posix.cc
+++ b/base/process_posix.cc
@@ -50,7 +50,7 @@ bool Process::EmptyWorkingSet() {
   return false;
 }
 
-int32 Process::pid() const {
+ProcessId Process::pid() const {
   if (process_ == 0)
     return 0;