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

src/objects.h

 // Copyright 2015 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.
 
 #ifndef V8_OBJECTS_H_
 #define V8_OBJECTS_H_
 
 #include <iosfwd>
 #include <memory>
 
@@ skipping 7954 matching lines @@
   DECLARE_VERIFIER(JSModuleNamespace)
 
   // The actual module whose namespace is being represented.
   DECL_ACCESSORS(module, Module)
 
   // Retrieve the value exported by [module] under the given [name]. If there is
   // no such export, return Just(undefined). If the export is uninitialized,
   // 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);
 };
 
 // A Module object is a mapping from export names to cells
 // This is still very much in flux.
 class Module : public Struct {
  public:
   DECLARE_CAST(Module)
@@ skipping 3747 matching lines @@
   }
 };
 
 
 }  // NOLINT, false-positive due to second-order macros.
 }  // NOLINT, false-positive due to second-order macros.
 
 #include "src/objects/object-macros-undef.h"
 
 #endif  // V8_OBJECTS_H_