Fixes a DCHECK in tools_menu_button_observer_bridge during tests

components/reading_list/core/reading_list_model_observer.h

Index: components/reading_list/core/reading_list_model_observer.h
diff --git a/components/reading_list/core/reading_list_model_observer.h b/components/reading_list/core/reading_list_model_observer.h
index 061d1cc3ac98c787f459b44be19050fbe5dccbed..a0ad2bf3951ba3a6378d36d2f71a4d72002245cd 100644
--- a/components/reading_list/core/reading_list_model_observer.h
+++ b/components/reading_list/core/reading_list_model_observer.h
@@ -32,6 +32,9 @@ class ReadingListModelObserver {
   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 The model is no longer valid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.