[mojo-bindings] Support reuse of native enum classes

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

 {#  TODO(yzshen): Make these templates more readable. #}
 
 {#  Computes the serialized size for the specified struct.
     |struct| is the struct definition.
     |input_field_pattern| should be a pattern that contains one string
     placeholder, for example, "input->%s", "p_%s". The placeholder will be
     substituted with struct field names to refer to the input fields.
     This macro is expanded to compute seriailized size for both:
     - user-defined structs: the input is an instance of the corresponding struct
       wrapper class.
@@ skipping 131 matching lines @@
 {%-   set last_checked_version = 0 %}
 {%-   for pf in struct.packed.packed_fields_in_ordinal_order %}
 {%-     set output_field = output_field_pattern|format(pf.field.name) %}
 {%-     set name = pf.field.name %}
 {%-     set kind = pf.field.kind %}
 {%-     if pf.min_version > last_checked_version %}
 {%-       set last_checked_version = pf.min_version %}
     if ({{input}}->header_.version < {{pf.min_version}})
       break;
 {%-     endif %}
-{%-     if kind|is_native_only_kind %}
+{%-     if kind|is_native_only_kind and kind|is_struct_kind %}
     if (!DeserializeNative_(
             {{input}}->{{name}}.ptr, &{{output_field}}, {{context}})) {
       {{success}} = false;
     }
+{%-     elif kind|is_native_only_kind and kind|is_enum_kind %}
+    {{output_field}} = static_cast<{{kind|cpp_wrapper_type}}>(
+        {{input}}->{{name}});
 {%-     elif kind|is_typemapped_kind %}
     if (!{{kind|get_name_for_kind}}_SerializerTraits_<{{kind|cpp_wrapper_type}}>
             ::Deserialize(
                 {{input}}->{{name}}.ptr, &{{output_field}}, {{context}})) {
       {{success}} = false;
     }
 {%-     elif kind|is_object_kind %}
 {%-       if kind|is_union_kind %}
     if (!Deserialize_(&{{input}}->{{name}}, &{{output_field}}, {{context}}))
       {{success}} = false;
@@ skipping 14 matching lines @@
         &{{output_field}},
         ({{context}})->router->CreateLocalEndpointHandle(mojo::internal::FetchAndReset(&{{input}}->{{name}})));
 {%-     elif kind|is_enum_kind %}
     {{output_field}} = static_cast<{{kind|cpp_wrapper_type}}>({{input}}->{{name}});
 {%-     else %}
     {{output_field}} = {{input}}->{{name}};
 {%-     endif %}
 {%-   endfor %}
   } while (false);
 {%- endmacro %}