Add DocumentShutdown{Notifier,Observer}

third_party/WebKit/Source/core/dom/DocumentShutdownNotifier.h

+// Copyright 2017 The Chromium 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 DocumentShutdownNotifier_h
+#define DocumentShutdownNotifier_h
+
+#include "platform/LifecycleNotifier.h"
+
+namespace blink {
+
+class Document;
+class DocumentShutdownObserver;
+
+class DocumentShutdownNotifier

[yosin_UTC9, 2017/06/16 01:06:21]

Could you add class level comment?

It seems DocumentShutdownNotifier needs to have CORE_EXPORT, see win bot's
compilation failure.

+    : public LifecycleNotifier<Document, DocumentShutdownObserver> {
+ protected:
+  DocumentShutdownNotifier();
+
+ private:
+  DISALLOW_COPY_AND_ASSIGN(DocumentShutdownNotifier);
+};
+
+}  // namespace blink
+
+#endif  // DocumentShutdownNotifier_h