Move platform_thread to base/threading and put in the base namespace. I left ...

base/singleton.h

 // Copyright (c) 2010 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef BASE_SINGLETON_H_
 #define BASE_SINGLETON_H_
 #pragma once
 
 #include "base/at_exit.h"
 #include "base/atomicops.h"
-#include "base/platform_thread.h"
 #include "base/third_party/dynamic_annotations/dynamic_annotations.h"
+#include "base/threading/platform_thread.h"
 #include "base/thread_restrictions.h"
 
 // Default traits for Singleton<Type>. Calls operator new and operator delete on
 // the object. Registers automatic deletion at process exit.
 // Overload if you need arguments or another memory allocation function.
 template<typename Type>
 struct DefaultSingletonTraits {
   // Allocates the object.
   static Type* New() {
     // The parenthesis is very important here; it forces POD type
@@ skipping 213 matching lines @@
     // - Has the object in BeingCreated state
     // - Already has the object created...
     // We know value != NULL.  It could be kBeingCreatedMarker, or a valid ptr.
     // Unless your constructor can be very time consuming, it is very unlikely
     // to hit this race.  When it does, we just spin and yield the thread until
     // the object has been created.
     while (true) {
       value = base::subtle::NoBarrier_Load(&instance_);
       if (value != kBeingCreatedMarker)
         break;
-      PlatformThread::YieldCurrentThread();
+      base::PlatformThread::YieldCurrentThread();
     }
 
     // See the corresponding HAPPENS_BEFORE above.
     ANNOTATE_HAPPENS_AFTER(&instance_);
     return reinterpret_cast<Type*>(value);
   }
 
   // Adapter function for use with AtExit().  This should be called single
   // threaded, so don't use atomic operations.
   // Calling OnExit while singleton is in use by other threads is a mistake.
   static void OnExit(void* unused) {
     // AtExit should only ever be register after the singleton instance was
     // created.  We should only ever get here with a valid instance_ pointer.
     Traits::Delete(
         reinterpret_cast<Type*>(base::subtle::NoBarrier_Load(&instance_)));
     instance_ = 0;
   }
   static base::subtle::AtomicWord instance_;
 };
 
 template <typename Type, typename Traits, typename DifferentiatingType>
 base::subtle::AtomicWord Singleton<Type, Traits, DifferentiatingType>::
     instance_ = 0;
 
 #endif  // BASE_SINGLETON_H_