Use PlatformThreadId, not int when dealing with thread ids.

base/non_thread_safe.h

Index: base/non_thread_safe.h
diff --git a/base/non_thread_safe.h b/base/non_thread_safe.h
index 45a352d581f6a2cecc743877ab539f19777aeef2..2791fe06e6b7f5a4a272a8e2bdb367c451bb0967 100644
--- a/base/non_thread_safe.h
+++ b/base/non_thread_safe.h
@@ -6,6 +6,7 @@
 #define BASE_NON_THREAD_SAFE_H__
 
 #include "base/logging.h"
+#include "base/platform_thread.h"
 
 // A helper class used to help verify that methods of a class are
 // called from the same thread.  One can inherit from this class and use
@@ -35,7 +36,7 @@ class NonThreadSafe {
   bool CalledOnValidThread() const;
 
  private:
-  int valid_thread_id_;
+  PlatformThreadId valid_thread_id_;
 };
 #else
 // Do nothing in release mode.