Add DocumentShutdown{Notifier,Observer}

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

Index: third_party/WebKit/Source/core/dom/DocumentShutdownObserver.h
diff --git a/third_party/WebKit/Source/core/dom/DocumentShutdownObserver.h b/third_party/WebKit/Source/core/dom/DocumentShutdownObserver.h
new file mode 100644
index 0000000000000000000000000000000000000000..d3e835893b5e48a7d2a6d7772cf2951f432385a4
--- /dev/null
+++ b/third_party/WebKit/Source/core/dom/DocumentShutdownObserver.h
@@ -0,0 +1,35 @@
+// 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 DocumentShutdownObserver_h
+#define DocumentShutdownObserver_h
+
+#include "core/CoreExport.h"
+#include "platform/LifecycleObserver.h"
+
+namespace blink {
+
+class Document;
+
+// This class is a base class for classes which observe Document shutdown
+// synchronously.
+// Note: this functionality is also provided by SynchronousMutationObserver,
+// but if you don't need to respond to the other events handled by that class,
+// using this class is more efficient.
+class CORE_EXPORT DocumentShutdownObserver
+    : public LifecycleObserver<Document, DocumentShutdownObserver> {
+ public:
+  // Called when detaching document.
+  virtual void ContextDestroyed(Document*);
+
+ protected:
+  DocumentShutdownObserver();
+
+ private:
+  DISALLOW_COPY_AND_ASSIGN(DocumentShutdownObserver);
+};
+
+}  // namespace blink
+
+#endif  // DocumentShutdownObserver_h