Mojo C++ bindings: add support for UnionTraits.

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 "base/optional.h"
 #include "mojo/public/cpp/bindings/array_traits.h"
 #include "mojo/public/cpp/bindings/enum_traits.h"
 #include "mojo/public/cpp/bindings/lib/template_util.h"
 #include "mojo/public/cpp/bindings/map_traits.h"
 #include "mojo/public/cpp/bindings/string_traits.h"
 #include "mojo/public/cpp/bindings/struct_traits.h"
+#include "mojo/public/cpp/bindings/union_traits.h"
 
 // This file is included by serialization implementation files to avoid circular
 // includes.
 // Users of the serialization funtions should include serialization.h (and also
 // wtf_serialization.h if necessary).
 
 namespace mojo {
 namespace internal {
 
 template <typename MojomType, typename MaybeConstUserType>
@@ skipping 88 matching lines @@
   if (!*output)
     output->emplace();
   return Deserialize<MojomType>(std::forward<DataType>(input), &output->value(),
                                 std::forward<Args>(args)...);
 }
 
 }  // namespace internal
 }  // namespace mojo
 
 #endif  // MOJO_PUBLIC_CPP_BINDINGS_LIB_SERIALIZATION_FORWARD_H_