[ios] Fix clean shutdown of Chrome on iOS.

ios/chrome/browser/ui/toolbar/tools_menu_button_observer_bridge.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 "ios/chrome/browser/ui/toolbar/tools_menu_button_observer_bridge.h"
 
 #include "base/logging.h"
 #include "base/memory/ptr_util.h"
 #include "components/reading_list/core/reading_list_model.h"
 #import "ios/chrome/browser/ui/toolbar/toolbar_tools_menu_button.h"
@@ skipping 45 matching lines @@
 - (void)readingListModelLoaded:(const ReadingListModel*)model {
   [self updateButtonWithModel:model];
 }
 
 - (void)readingListModelDidApplyChanges:(const ReadingListModel*)model {
   [self updateButtonWithModel:model];
 }
 
 - (void)readingListModelBeingDeleted:(const ReadingListModel*)model {
   DCHECK(model == _model);
+  _modelBridge.reset();
   _model = nullptr;
 }
 
 - (void)readingListModel:(const ReadingListModel*)model
              didAddEntry:(const GURL&)url
              entrySource:(reading_list::EntrySource)source {
   if (source == reading_list::ADDED_VIA_CURRENT_APP)
     [_button triggerAnimation];
 }
 
 @end