Add DocumentShutdown{Notifier,Observer}

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

Index: third_party/WebKit/Source/core/dom/DocumentShutdownNotifier.cpp
diff --git a/third_party/WebKit/Source/core/dom/DocumentShutdownNotifier.cpp b/third_party/WebKit/Source/core/dom/DocumentShutdownNotifier.cpp
new file mode 100644
index 0000000000000000000000000000000000000000..5ebeea365c29411db758d720ed9ca67fb7347b47
--- /dev/null
+++ b/third_party/WebKit/Source/core/dom/DocumentShutdownNotifier.cpp
@@ -0,0 +1,14 @@
+// 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.
+
+#include "core/dom/DocumentShutdownNotifier.h"
+
+#include "core/dom/Document.h"
+#include "core/dom/DocumentShutdownObserver.h"
+
+namespace blink {
+
+DocumentShutdownNotifier::DocumentShutdownNotifier() = default;
+
+}  // namespace blink