Fixes a DCHECK in tools_menu_button_observer_bridge during tests

components/reading_list/ios/reading_list_model_bridge_observer.mm

 // 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.
 
 #import "components/reading_list/ios/reading_list_model_bridge_observer.h"
 
 #include "components/reading_list/core/reading_list_entry.h"
 #include "components/reading_list/core/reading_list_model.h"
 
 ReadingListModelBridge::ReadingListModelBridge(
     id<ReadingListModelBridgeObserver> observer,
     ReadingListModel* model)
     : observer_(observer), model_(model) {
   DCHECK(model);
   model_->AddObserver(this);
 }
 
 ReadingListModelBridge::~ReadingListModelBridge() {
   if (model_) {
     model_->RemoveObserver(this);
   }
 }
 
 void ReadingListModelBridge::ReadingListModelLoaded(
     const ReadingListModel* model) {
   [observer_ readingListModelLoaded:model];
 }
 
+void ReadingListModelBridge::ReadingListModelBeingShutdown(
+    const ReadingListModel* model) {
+  if ([observer_
+          respondsToSelector:@selector(readingListModelBeingShutdown:)]) {
+    [observer_ readingListModelBeingShutdown:model];
+  }
+}
+
 void ReadingListModelBridge::ReadingListModelBeingDeleted(
     const ReadingListModel* model) {
   if ([observer_ respondsToSelector:@selector(readingListModelBeingDeleted:)]) {
     [observer_ readingListModelBeingDeleted:model];
   }
   model_ = nullptr;
 }
 
 void ReadingListModelBridge::ReadingListWillRemoveEntry(
     const ReadingListModel* model,
@@ skipping 56 matching lines @@
 }
 
 void ReadingListModelBridge::ReadingListWillUpdateEntry(
     const ReadingListModel* model,
     const GURL& url) {
   if ([observer_
           respondsToSelector:@selector(readingListModel:willUpdateEntry:)]) {
     [observer_ readingListModel:model willUpdateEntry:url];
   }
 }