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

src/objects.h

Index: src/objects.h
diff --git a/src/objects.h b/src/objects.h
index 9daa50e1f632ae1d5c2bad68fbbf0a57802d4504..1c8030c68bfa7c52aeb37163f61a3a35537fb648 100644
--- a/src/objects.h
+++ b/src/objects.h
@@ -7972,8 +7972,16 @@ class JSModuleNamespace : public JSObject {
   // schedule an exception and return Nothing.
   MUST_USE_RESULT MaybeHandle<Object> GetExport(Handle<String> name);
 
+  // In-object fields.
+  enum {
+    kToStringTagFieldIndex,
+    kInObjectFieldCount,
+  };
+
   static const int kModuleOffset = JSObject::kHeaderSize;
-  static const int kSize = kModuleOffset + kPointerSize;
+  static const int kHeaderSize = kModuleOffset + kPointerSize;
+
+  static const int kSize = kHeaderSize + kPointerSize * kInObjectFieldCount;
 
  private:
   DISALLOW_IMPLICIT_CONSTRUCTORS(JSModuleNamespace);