Mojo C++ bindings: support mapping mojo string to WTF::String.

mojo/public/cpp/bindings/lib/serialization_forward.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.
 
 #ifndef MOJO_PUBLIC_CPP_BINDINGS_LIB_SERIALIZATION_FORWARD_H_
 #define MOJO_PUBLIC_CPP_BINDINGS_LIB_SERIALIZATION_FORWARD_H_
 
 #include <stddef.h>
 
 #include "mojo/public/cpp/bindings/lib/string_serialization.h"
+#include "mojo/public/cpp/bindings/lib/wtf_string_serialization.h"
 
 // This file is included by serialization implementation files to avoid circular
 // includes.
-// Users of the serialization funtions should include serialization.h.
+// Users of the serialization funtions should include serialization.h (and also
+// wtf_serialization.h if necessary).
 
 namespace mojo {
 
 template <typename T>
 class Array;
 
 template <typename K, typename V>
 class Map;
 
 namespace internal {
@@ skipping 69 matching lines @@
           typename MapValue,
           typename DataKey,
           typename DataValue>
 inline bool Deserialize_(internal::Map_Data<DataKey, DataValue>* input,
                          Map<MapKey, MapValue>* output,
                          internal::SerializationContext* context);
 
 }  // namespace mojo
 
 #endif  // MOJO_PUBLIC_CPP_BINDINGS_LIB_SERIALIZATION_FORWARD_H_