Add DocumentShutdown{Notifier,Observer}

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

Index: third_party/WebKit/Source/core/dom/DocumentShutdownObserver.cpp
diff --git a/third_party/WebKit/Source/core/dom/DocumentShutdownObserver.cpp b/third_party/WebKit/Source/core/dom/DocumentShutdownObserver.cpp
new file mode 100644
index 0000000000000000000000000000000000000000..a6be27e57839f602fabc232c3ed6b8df1114a31c
--- /dev/null
+++ b/third_party/WebKit/Source/core/dom/DocumentShutdownObserver.cpp
@@ -0,0 +1,15 @@
+// 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/DocumentShutdownObserver.h"
+
+#include "core/dom/Document.h"
+#include "core/dom/DocumentShutdownNotifier.h"
+
+namespace blink {
+
+DocumentShutdownObserver::DocumentShutdownObserver()
+    : LifecycleObserver(nullptr) {}
+
+}  // namespace blink