Mojo C++ bindings: add test cases for union custom typemapping.

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) %}
 
 template <>
 struct {{export_attribute}} UnionTraits<{{mojom_type}}, {{mojom_type}}Ptr> {
   static bool IsNull(const {{mojom_type}}Ptr& input) { return !input; }
   static void SetToNull({{mojom_type}}Ptr* output) { output->reset(); }
 
   static {{mojom_type}}::Tag GetTag(const {{mojom_type}}Ptr& input) {
     return input->which();
@@ skipping 54 matching lines @@
             in_{{name}}, context);
 {%-     endif %}
 {%-   endif %}
         break;
       }
 {%- endfor %}
     }
     return size;
   }
 
-  static void Serialize({{mojom_type}}Ptr& input,
+  static void Serialize(MaybeConstUserType& input,
                         Buffer* buffer,
                         {{data_type}}** output,
                         bool inlined,
                         SerializationContext* context) {
     if (CallIsNullIfExists<Traits>(input)) {
       if (inlined)
         (*output)->set_null();
       else
         *output = nullptr;
       return;
@@ skipping 63 matching lines @@
 {%-   endif %}
         break;
       }
 {%- endfor %}
     }
 
     CustomContextHelper<Traits>::TearDown(input, custom_context);
   }
 
   static bool Deserialize({{data_type}}* input,
-                          {{mojom_type}}Ptr* output,
+                          UserType* output,
                           SerializationContext* context) {
     if (!input || input->is_null())
       return CallSetToNullIfExists<Traits>(output);
 
     {{mojom_type}}DataView data_view(input, context);
     return Traits::Read(data_view, output);
   }
 };
 
 }  // namespace internal