Use PlatformThreadId, not int when dealing with thread ids.

base/simple_thread.h

Index: base/simple_thread.h
diff --git a/base/simple_thread.h b/base/simple_thread.h
index edcbfbe636d4fd0692cb6c0ebe0d792a77e5dac8..17a2f6d1d3c4f7b1ef06124bc3152ab1c472bd9d 100644
--- a/base/simple_thread.h
+++ b/base/simple_thread.h
@@ -98,7 +98,7 @@ class SimpleThread : public PlatformThread::Delegate {
   std::string name() { return name_; }
 
   // Return the thread id, only valid after Start().
-  int tid() { return tid_; }
+  PlatformThreadId tid() { return tid_; }
 
   // Return True if Start() has ever been called.
   bool HasBeenStarted() { return event_.IsSignaled(); }
@@ -112,7 +112,7 @@ class SimpleThread : public PlatformThread::Delegate {
   const Options options_;
   PlatformThreadHandle thread_;  // PlatformThread handle, invalid after Join!
   WaitableEvent event_;          // Signaled if Start() was ever called.
-  int tid_;                      // The backing thread's id.
+  PlatformThreadId tid_;         // The backing thread's id.
   bool joined_;                  // True if Join has been called.
 };