Move files in wtf/ to platform/wtf/ (Part 4).

third_party/WebKit/Source/wtf/SpinLock.h

-// Copyright 2016 The Chromium Authors. All rights reserved.
+// Copyright 2017 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 WTF_SpinLock_h
-#define WTF_SpinLock_h
+#include "platform/wtf/SpinLock.h"
 
-#include "base/allocator/partition_allocator/spin_lock.h"
-#include "base/macros.h"
-
-namespace WTF {
-
-// WTF::SpinLock is base::SpinLock. See
-// base/allocator/partition_allocator/spin_lock.h for documentation.
-using SpinLock = base::subtle::SpinLock;
-
-}  // namespace WTF
-
-using WTF::SpinLock;
-
-#endif  // WTF_SpinLock_h
+// The contents of this header was moved to platform/wtf as part of
+// WTF migration project. See the following post for details:
+// https://groups.google.com/a/chromium.org/d/msg/blink-dev/tLdAZCTlcAA/bYXVT8gYCAAJ