Support exporting Mojo bindings for the component build.

mojo/public/tools/bindings/generators/cpp_templates/union_serialization_declaration.tmpl

 {%- set mojom_type = union|get_qualified_name_for_kind %}
 {%- set data_type = union|get_qualified_name_for_kind(internal=True) %}
 
 namespace internal {
 
 template <typename MojomType>
 struct UnionSerializerImpl;
 
 template <>
-struct UnionSerializerImpl<{{mojom_type}}Ptr> {
+struct {{export_attribute}} UnionSerializerImpl<{{mojom_type}}Ptr> {
   static size_t PrepareToSerialize({{mojom_type}}Ptr& input,
                                    bool inlined,
                                    SerializationContext* context);
 
   static void Serialize({{mojom_type}}Ptr& input,
                         Buffer* buffer,
                         {{data_type}}** output,
                         bool inlined,
                         SerializationContext* context);
 
   static bool Deserialize({{data_type}}* input,
                           {{mojom_type}}Ptr* output,
                           SerializationContext* context);
 };
 
 template <typename MaybeConstUserType>
-struct Serializer<{{mojom_type}}Ptr, MaybeConstUserType>
+struct {{export_attribute}} Serializer<{{mojom_type}}Ptr, MaybeConstUserType>
     : public UnionSerializerImpl<{{mojom_type}}Ptr> {
   using UserType = typename std::remove_const<MaybeConstUserType>::type;
 
   static_assert(std::is_same<MaybeConstUserType, UserType>::value,
                 "Only support serialization of non-const Unions.");
   static_assert(std::is_same<UserType, {{mojom_type}}Ptr>::value,
                 "Custom mapping of mojom union is not supported.");
 };
 
 }  // namespace internal