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

src/objects-printer.cc

Index: src/objects-printer.cc
diff --git a/src/objects-printer.cc b/src/objects-printer.cc
index f1ca6212b07575a98d81e17a3f8dbb907d18ab8c..f6d89b89c573cdfa1631898e3b12b6c76401096c 100644
--- a/src/objects-printer.cc
+++ b/src/objects-printer.cc
@@ -1289,9 +1289,9 @@ void Module::ModulePrint(std::ostream& os) {  // NOLINT
 }
 
 void JSModuleNamespace::JSModuleNamespacePrint(std::ostream& os) {  // NOLINT
-  HeapObject::PrintHeader(os, "JSModuleNamespace");
+  JSObjectPrintHeader(os, this, "JSModuleNamespace");
   os << "\n - module: " << Brief(module());
-  os << "\n";
+  JSObjectPrintBody(os, this);
 }
 
 void PrototypeInfo::PrototypeInfoPrint(std::ostream& os) {  // NOLINT