Add DocumentShutdown{Notifier,Observer}

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

 // Copyright 2016 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 SynchronousMutationObserver_h
 #define SynchronousMutationObserver_h
 
 #include "base/macros.h"
 #include "core/CoreExport.h"
 #include "platform/LifecycleObserver.h"
 
 namespace blink {
 
 class CharacterData;
 class ContainerNode;
 class Document;
 class NodeWithIndex;
 class Text;
 
 // This class is a base class for classes which observe DOM tree mutation
 // synchronously. If you want to observe DOM tree mutation asynchronously see
 // MutationObserver Web API.
+// Note: if you only need to observe Document shutdown,
+// DocumentShutdownObserver provides this same functionality more efficiently
+// (since it doesn't observe the other events).
 //
 // TODO(yosin): Following classes should be derived from this class to
 // simplify Document class.
 //  - DragCaret
 //  - DocumentMarkerController
 //  - EventHandler
 //  - FrameCaret
 //  - InputMethodController
 //  - SelectionController
 //  - Range set
@@ skipping 44 matching lines @@
  protected:
   SynchronousMutationObserver();
 
  private:
   DISALLOW_COPY_AND_ASSIGN(SynchronousMutationObserver);
 };
 
 }  // namespace blink
 
 #endif  // SynchronousMutationObserver_h