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

third_party/WebKit/Source/platform/wtf/Optional.h

 // Copyright 2015 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 "base/optional.h"
-#include "wtf/TypeTraits.h"
+#include "platform/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