Fixes a DCHECK in tools_menu_button_observer_bridge during tests

components/reading_list/core/reading_list_model_observer.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 COMPONENTS_READING_LIST_CORE_READING_LIST_MODEL_OBSERVER_H_
 #define COMPONENTS_READING_LIST_CORE_READING_LIST_MODEL_OBSERVER_H_
 
 #include <set>
 #include <vector>
 
@@ skipping 14 matching lines @@
   // once before ReadingListModelCompletedBatchUpdates, even if several updates
   // are taking place at the same time.
   virtual void ReadingListModelBeganBatchUpdates(
       const ReadingListModel* model) {}
 
   // Invoked when the batch updates have completed. This is called once all
   // batch updates are completed.
   virtual void ReadingListModelCompletedBatchUpdates(
       const ReadingListModel* model) {}
 
+  // Invoked before the destruction of the model.
+  virtual void ReadingListModelBeingShutdown(const ReadingListModel* model) {}
+
   // Invoked from the destructor of the model. The model is no longer valid
   // after this call. There is no need to call RemoveObserver on the model from
   // here, as the observers are automatically deleted.
   virtual void ReadingListModelBeingDeleted(const ReadingListModel* model) {}
 
   // Invoked when elements are about to be removed from the read or unread list.
   virtual void ReadingListWillRemoveEntry(const ReadingListModel* model,
                                           const GURL& url) {}
   // Invoked when elements |MarkEntryUpdated| is called on an entry. This means
   // that the order of the entry may change and read/unread list may change
@@ skipping 29 matching lines @@
   virtual void ReadingListDidApplyChanges(ReadingListModel* model) {}
 
  protected:
   ReadingListModelObserver() {}
   virtual ~ReadingListModelObserver() {}
 
   DISALLOW_COPY_AND_ASSIGN(ReadingListModelObserver);
 };
 
 #endif  // COMPONENTS_READING_LIST_CORE_READING_LIST_MODEL_OBSERVER_H_