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 "core/CoreExport.h"
+#include "platform/LifecycleNotifier.h"
+
+namespace blink {
+
+class Document;
+class DocumentShutdownObserver;
+
+// Sibling class of DocumentShutdownObserver; implemented by Document to notify
+// subclasses of DocumentShutdownObserver of Document shutdown.
+class CORE_EXPORT DocumentShutdownNotifier
+    : public LifecycleNotifier<Document, DocumentShutdownObserver> {
+ protected:
+  DocumentShutdownNotifier();
+
+ private:
+  DISALLOW_COPY_AND_ASSIGN(DocumentShutdownNotifier);
+};
+
+}  // namespace blink
+
+#endif  // DocumentShutdownNotifier_h