Revert of Move WTF::SpinLock to base::SpinLock.

base/synchronization/spin_lock.h

-// Copyright (c) 2013 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_SYNCHRONIZATION_SPIN_LOCK_H
-#define BASE_SYNCHRONIZATION_SPIN_LOCK_H
-
-#include <atomic>
-#include <memory>
-#include <mutex>
-
-#include "base/base_export.h"
-#include "base/compiler_specific.h"
-
-// Spinlock is a simple spinlock class based on the standard CPU primitive of
-// atomic increment and decrement of an int at a given memory address. These are
-// intended only for very short duration locks and assume a system with multiple
-// cores. For any potentially longer wait you should use a real lock, such as
-// |base::Lock|.
-//
-// |SpinLock|s MUST be globals. Using them as (e.g.) struct/class members will
-// result in an uninitialized lock, which is dangerously incorrect.
-
-namespace base {
-namespace subtle {
-
-class SpinLock {
- public:
-  using Guard = std::lock_guard<SpinLock>;
-
-  ALWAYS_INLINE void lock() {
-    static_assert(sizeof(lock_) == sizeof(int),
-                  "int and lock_ are different sizes");
-    if (LIKELY(!lock_.exchange(true, std::memory_order_acquire)))
-      return;
-    LockSlow();
-  }
-
-  ALWAYS_INLINE void unlock() { lock_.store(false, std::memory_order_release); }
-
- private:
-  // This is called if the initial attempt to acquire the lock fails. It's
-  // slower, but has a much better scheduling and power consumption behavior.
-  BASE_EXPORT void LockSlow();
-
-  std::atomic_int lock_;
-};
-
-}  // namespace subtle
-}  // namespace base
-
-#endif  // BASE_SYNCHRONIZATION_SPIN_LOCK_H