[modules] Define @@toStringTag on namespace object as a field.

src/bootstrapper.cc

 // Copyright 2014 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "src/bootstrapper.h"
 
 #include "src/accessors.h"
 #include "src/api-natives.h"
 #include "src/base/ieee754.h"
 #include "src/code-stubs.h"
@@ skipping 2549 matching lines @@
   {  // -- J S M o d u l e N a m e s p a c e
     Handle<Map> map =
         factory->NewMap(JS_MODULE_NAMESPACE_TYPE, JSModuleNamespace::kSize);
     Map::SetPrototype(map, isolate->factory()->null_value());
     Map::EnsureDescriptorSlack(map, 1);
     native_context()->set_js_module_namespace_map(*map);
 
     {  // Install @@toStringTag.
       PropertyAttributes attribs =
           static_cast<PropertyAttributes>(DONT_DELETE | DONT_ENUM | READ_ONLY);
-      Descriptor d = Descriptor::DataConstant(
-          factory->to_string_tag_symbol(),
-          factory->NewStringFromAsciiChecked("Module"), attribs);
+      Descriptor d =
+          Descriptor::DataField(factory->to_string_tag_symbol(),
+                                JSModuleNamespace::kToStringTagFieldIndex,
+                                attribs, Representation::Tagged());
       map->AppendDescriptor(&d);
     }
+
+    map->SetInObjectProperties(JSModuleNamespace::kInObjectFieldCount);
   }
 
   {  // -- I t e r a t o r R e s u l t
     Handle<Map> map =
         factory->NewMap(JS_OBJECT_TYPE, JSIteratorResult::kSize);
     Map::SetPrototype(map, isolate->initial_object_prototype());
     Map::EnsureDescriptorSlack(map, 2);
 
     {  // value
       Descriptor d = Descriptor::DataField(factory->value_string(),
@@ skipping 2157 matching lines @@
 }
 
 
 // Called when the top-level V8 mutex is destroyed.
 void Bootstrapper::FreeThreadResources() {
   DCHECK(!IsActive());
 }
 
 }  // namespace internal
 }  // namespace v8