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

src/objects-inl.h

 // Copyright 2012 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.
 //
 // Review notes:
 //
 // - The use of macros in these inline functions may seem superfluous
 // but it is absolutely needed to make sure gcc generates optimal
 // code. gcc is not happy when attempting to inline too deep.
 //
@@ skipping 2200 matching lines @@
       return JSObject::kHeaderSize;
     case JS_MESSAGE_OBJECT_TYPE:
       return JSMessageObject::kSize;
     case JS_ARGUMENTS_TYPE:
       return JSArgumentsObject::kHeaderSize;
     case JS_ERROR_TYPE:
       return JSObject::kHeaderSize;
     case JS_STRING_ITERATOR_TYPE:
       return JSStringIterator::kSize;
     case JS_MODULE_NAMESPACE_TYPE:
-      return JSModuleNamespace::kSize;
+      return JSModuleNamespace::kHeaderSize;
     default:
       if (type >= FIRST_ARRAY_ITERATOR_TYPE &&
           type <= LAST_ARRAY_ITERATOR_TYPE) {
         return JSArrayIterator::kSize;
       }
       UNREACHABLE();
       return 0;
   }
 }
 
@@ skipping 6216 matching lines @@
 #undef WRITE_INT64_FIELD
 #undef READ_BYTE_FIELD
 #undef WRITE_BYTE_FIELD
 #undef NOBARRIER_READ_BYTE_FIELD
 #undef NOBARRIER_WRITE_BYTE_FIELD
 
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_OBJECTS_INL_H_