Move files in wtf/ to platform/wtf/ (Part 3).

third_party/WebKit/Source/wtf/Optional.h

-// Copyright 2015 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 Optional_h
-#define Optional_h
+#include "platform/wtf/Optional.h"
 
-#include "base/optional.h"
-#include "wtf/TypeTraits.h"
-
-namespace WTF {
-
-// WTF::Optional is base::Optional. See base/optional.h for documentation.
-//
-// A clang plugin enforces that garbage collected types are not allocated
-// outside of the heap, similarly we enforce that one doesn't create garbage
-// collected types nested inside an Optional.
-template <typename T>
-using Optional = typename std::enable_if<!IsGarbageCollectedType<T>::value,
-                                         base::Optional<T>>::type;
-
-constexpr base::nullopt_t nullopt = base::nullopt;
-constexpr base::in_place_t in_place = base::in_place;
-
-}  // namespace WTF
-
-using WTF::Optional;
-
-#endif  // Optional_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