Mojo C++ bindings: support enum validation.

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

 {%- macro validate_not_null_ptr(field_expr, field, object_name) %}
 if (!{{field_expr}}->offset) {
   ReportValidationError(
       mojo::internal::VALIDATION_ERROR_UNEXPECTED_NULL_POINTER,
       "null {{field.name}} field in {{object_name}}");
   return false;
 }
 {%- endmacro %}
 
 {%- macro validate_encoded_ptr(field_expr) %}
@@ skipping 23 matching lines @@
         "invalid {{field.name}} field in {{object_name}}");
     return false;
   }
 {%-   endif %}
   if (!bounds_checker->ClaimHandle({{field.name}}_handle)) {
     ReportValidationError(mojo::internal::VALIDATION_ERROR_ILLEGAL_HANDLE);
     return false;
   }
 {%- endmacro -%}
 
+{%- macro validate_enum(field_expr, field) -%}
+if (!mojo::internal::ValidateEnum(*{{field_expr}}))
+  return false;
+{%- endmacro -%}
+
 {%- macro validate_union_field(field, union) %}
 {%-   set field_expr = "(reinterpret_cast<const "
     ~ field.kind|cpp_union_field_type
     ~ "*>(&object->data.f_"
     ~ field.name
     ~ "))" -%}
 {%-   if field.kind|is_object_kind -%}
 {%-     if not field.kind|is_nullable_kind -%}
 {{        validate_not_null_ptr(field_expr, field, union.name) }}
 {%-     endif %}
 {{      validate_encoded_ptr(field_expr) }}
 {%-   endif %}
 
 {%-   if field.kind|is_array_kind or field.kind|is_string_kind -%}
 {{      validate_array_or_string(field_expr, field) }}
 {%-   endif %}
 
 {%-   if field.kind|is_any_handle_kind -%}
 {{      validate_handle(field_expr, field, union.name) }}
 {%-   endif %}
+
+{%-   if field.kind|is_enum_kind -%}
+{{      validate_enum(field_expr, field) }}
+{%-   endif %}
 return true;
 {%- endmacro %}