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

base/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"
-
-// This code is only done in debug builds.
-#ifndef NDEBUG
-
-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();
-}
-
-#endif  // NDEBUG