[mojo] Add pickling support for native-only structs

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

 {%- set class_name = struct.name ~ "_Data" %}
 
 {#- Validates the specified struct field, which is supposed to be an object
     (struct/array/string/map/union).
     This macro is expanded by the Validate() method. #}
 {%- macro _validate_object(struct, packed_field) %}
 {%-   set name = packed_field.field.name %}
 {%-   set kind = packed_field.field.kind %}
 {%-   if not kind|is_nullable_kind %}
 {%-     if kind|is_union_kind %}
   if (!mojo::internal::ValidateInlinedUnionNonNullable(
           object->{{name}}, "null {{name}} field in {{struct.name}} struct")) {
     return false;
   }
 {%-     else %}
   if (!mojo::internal::ValidatePointerNonNullable(
           object->{{name}}, "null {{name}} field in {{struct.name}} struct")) {
     return false;
   }
 {%-     endif %}
 {%-   endif %}
-{%-   if kind|is_array_kind or kind|is_string_kind %}
+{%-   if kind|is_array_kind or kind|is_string_kind or
+         kind|is_native_only_kind %}
   const mojo::internal::ArrayValidateParams {{name}}_validate_params(
       {{kind|get_array_validate_params_ctor_args|indent(6)}});
   if (!mojo::internal::ValidateArray(object->{{name}}, bounds_checker,
                                      &{{name}}_validate_params)) {
     return false;
   }
 {%-   elif kind|is_map_kind %}
   const mojo::internal::ArrayValidateParams {{name}}_validate_params(
       {{kind.value_kind|get_map_validate_params_ctor_args|indent(6)}});
   if (!mojo::internal::ValidateMap(object->{{name}}, bounds_checker,
@@ skipping 171 matching lines @@
   mojo::internal::DecodeHandle(&{{name}}, handles);
 {%-     endif %}
 {%-   endif %}
 {%- endfor %}
 }
 
 {{class_name}}::{{class_name}}() {
   header_.num_bytes = sizeof(*this);
   header_.version = {{struct.versions[-1].version}};
 }