Mojo C++ bindings: extract code shared by different variants.

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

 {#- Validates the specified field, which is supposed to be an object
     (struct/array/string/map/union). If it is a union, |union_is_inlined|
     indicates whether the union is inlined. (Nested unions are not inlined.)
     This macro is expanded by the Validate() method. #}
 {%- macro validate_object(field, field_expr, object_name, union_is_inlined) %}
 {%-   set name = field.name %}
 {%-   set kind = field.kind %}
 {%-   if not kind|is_nullable_kind %}
 {%-     if kind|is_union_kind and union_is_inlined %}
   if (!mojo::internal::ValidateInlinedUnionNonNullable(
@@ skipping 48 matching lines @@
 {%-   endif %}
   if (!mojo::internal::ValidateHandleOrInterface({{field_expr}},
                                                  validation_context)) {
     return false;
   }
 {%- endmacro %}
 
 {#- Validates the specified field, which is supposed to be an enum.
     This macro is expanded by the Validate() method. #}
 {%- macro validate_enum(field, field_expr) %}
-  if (!{{field.kind|get_qualified_name_for_kind(internal=True)}}
+  if (!{{field.kind|get_qualified_name_for_kind(internal=True,flatten_nested_kind=True)}}
         ::Validate({{field_expr}}, validation_context))
     return false;
 {%- endmacro %}
 
 {%- macro validate_field(field, field_expr, object_name, union_is_inlined) %}
 {%-   if field.kind|is_object_kind -%}
 {{validate_object(field, field_expr, object_name, union_is_inlined)}}
 {%-   elif field.kind|is_any_handle_or_interface_kind -%}
 {{validate_handle_or_interface(field, field_expr, object_name)}}
 {%-   elif field.kind|is_enum_kind %}
 {{validate_enum(field, field_expr)}}
 {%-   endif %}
 {%- endmacro %}