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

base/threading/thread_checker.cc

 // 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.
 
-#include "base/thread_checker.h"
+#include "base/threading/thread_checker.h"
 
 // This code is only done in debug builds.
 #ifndef NDEBUG
 
+namespace base {
+
 ThreadChecker::ThreadChecker() : valid_thread_id_(kInvalidThreadId) {
   EnsureThreadIdAssigned();
 }
 
 ThreadChecker::~ThreadChecker() {}
 
 bool ThreadChecker::CalledOnValidThread() const {
   EnsureThreadIdAssigned();
   AutoLock auto_lock(lock_);
   return valid_thread_id_ == PlatformThread::CurrentId();
 }
 
 void ThreadChecker::DetachFromThread() {
   AutoLock auto_lock(lock_);
   valid_thread_id_ = kInvalidThreadId;
 }
 
 void ThreadChecker::EnsureThreadIdAssigned() const {
   AutoLock auto_lock(lock_);
   if (valid_thread_id_ != kInvalidThreadId)
     return;
   valid_thread_id_ = PlatformThread::CurrentId();
 }
 
+}  // namespace base
+
 #endif  // NDEBUG