Mojo C++ bindings: introduce an optional array to store transferred interface IDs in messages.

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

 {%- set data_view = union|get_qualified_name_for_kind ~ "DataView" %}
 {%- set data_type = union|get_qualified_name_for_kind(internal=True) %}
 
 namespace internal {
 
 template <typename MaybeConstUserType>
 struct Serializer<{{data_view}}, MaybeConstUserType> {
   using UserType = typename std::remove_const<MaybeConstUserType>::type;
   using Traits = UnionTraits<{{data_view}}, UserType>;
 
   static size_t PrepareToSerialize(MaybeConstUserType& input,
                                    bool inlined,
                                    SerializationContext* context) {
     size_t size = inlined ? 0 : sizeof({{data_type}});
 
     if (CallIsNullIfExists<Traits>(input))
       return size;
 
     void* custom_context = CustomContextHelper<Traits>::SetUp(input, context);
     ALLOW_UNUSED_LOCAL(custom_context);
 
     switch (CallWithContext(Traits::GetTag, input, custom_context)) {
 {%- for field in union.fields %}
 {%-   set name = field.name %}
       case {{data_view}}::Tag::{{name|upper}}: {
-{%-   if field.kind|is_object_kind %}
+{%-   if field.kind|is_object_kind or field.kind|is_associated_kind %}
 {%-     set kind = field.kind %}
 {%-     set serializer_type = kind|unmapped_type_for_serializer %}
         decltype(CallWithContext(Traits::{{name}}, input, custom_context))
             in_{{name}} = CallWithContext(Traits::{{name}}, input,
                                           custom_context);
 {%-     if kind|is_union_kind %}
         size += mojo::internal::PrepareToSerialize<{{serializer_type}}>(
             in_{{name}}, false, context);
 {%-     else %}
         size += mojo::internal::PrepareToSerialize<{{serializer_type}}>(
@@ skipping 95 matching lines @@
                           SerializationContext* context) {
     if (!input || input->is_null())
       return CallSetToNullIfExists<Traits>(output);
 
     {{data_view}} data_view(input, context);
     return Traits::Read(data_view, output);
   }
 };
 
 }  // namespace internal