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

src/bootstrapper.cc

Index: src/bootstrapper.cc
diff --git a/src/bootstrapper.cc b/src/bootstrapper.cc
index d26a27bf645ebfd9717bfc5b262b9687612319ad..959721308ba06284b04da758e14a58f41ea4a54f 100644
--- a/src/bootstrapper.cc
+++ b/src/bootstrapper.cc
@@ -2567,11 +2567,14 @@ void Genesis::InitializeGlobal(Handle<JSGlobalObject> global_object,
     {  // 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