Remove unneeded platform_thread.h includes. Reduces number of includes from 5...

base/thread_collision_warner.h

 // Copyright (c) 2008 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 THREAD_COLLISION_WARNER_H_
 #define THREAD_COLLISION_WARNER_H_
 
 #include <memory>
 
 #include "base/atomicops.h"
-#include "base/platform_thread.h"
 
 // A helper class alongside macros to be used to verify assumptions about thread
 // safety of a class.
 //
 // Example: Queue implementation non thread-safe but still usable if clients
 //          are synchronized somehow.
 //
 //          In this case the macro DFAKE_SCOPED_LOCK has to be
 //          used, it checks that if a thread is inside the push/pop then
 //          noone else is still inside the pop/push
@@ skipping 212 matching lines @@
   // Here only for class unit tests purpose, during the test I need to not
   // DCHECK but notify the collision with something else.
   AsserterBase* asserter_;
 
   DISALLOW_COPY_AND_ASSIGN(ThreadCollisionWarner);
 };
 
 }  // namespace base
 
 #endif  // THREAD_COLLISION_WARNER_H_