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

third_party/WebKit/Source/wtf/SpinLock.h

-// Copyright 2016 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.
+/*
+ * Copyright (C) 2013 Google Inc. All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are
+ * met:
+ *
+ *     * Redistributions of source code must retain the above copyright
+ * notice, this list of conditions and the following disclaimer.
+ *     * Redistributions in binary form must reproduce the above
+ * copyright notice, this list of conditions and the following disclaimer
+ * in the documentation and/or other materials provided with the
+ * distribution.
+ *     * Neither the name of Google Inc. nor the names of its
+ * contributors may be used to endorse or promote products derived from
+ * this software without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+ * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+ * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+ * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+ * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+ * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+ * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+ * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
 
 #ifndef WTF_SpinLock_h
 #define WTF_SpinLock_h
 
-#include "base/macros.h"
-#include "base/synchronization/spin_lock.h"
+#include "wtf/Compiler.h"
+#include "wtf/WTFExport.h"
+#include <atomic>
+#include <memory>
+#include <mutex>
+
+// DESCRIPTION
+// 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 be using a real lock.
 
 namespace WTF {
 
-// WTF::SpinLock is base::SpinLock. See base/synchronization/spin_lock.h for
-// documentation.
-using SpinLock = base::subtle::SpinLock;
+class SpinLock {
+ public:
+  using Guard = std::lock_guard<SpinLock>;
+
+  ALWAYS_INLINE void lock() {
+    static_assert(sizeof(m_lock) == sizeof(int),
+                  "int and m_lock are different sizes");
+    if (LIKELY(!m_lock.exchange(true, std::memory_order_acquire)))
+      return;
+    lockSlow();
+  }
+
+  ALWAYS_INLINE void unlock() {
+    m_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.
+  WTF_EXPORT void lockSlow();
+
+  std::atomic_int m_lock;
+};
 
 }  // namespace WTF
 
 using WTF::SpinLock;
 
 #endif  // WTF_SpinLock_h