Fixes a DCHECK in tools_menu_button_observer_bridge during tests

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 43 matching lines @@
 #pragma mark - ReadingListModelBridgeObserver
 
 - (void)readingListModelLoaded:(const ReadingListModel*)model {
   [self updateButtonWithModel:model];
 }
 
 - (void)readingListModelDidApplyChanges:(const ReadingListModel*)model {
   [self updateButtonWithModel:model];
 }
 
+- (void)readingListModelBeingShutdown:(const ReadingListModel*)model {
+  DCHECK(model == _model);
+}

[noyau (Ping after 24h), 2017/05/09 13:27:10]

Is this method really necessary? In C++ this would be removed at link time, but
in objectiveC the empty method will stay here. Just for a DCHECK that is very
unlikely to be hit.

[pkl (ping after 24h if needed), 2017/05/18 04:51:04]

Removing this method since it is not doing anything really useful.

+
 - (void)readingListModelBeingDeleted:(const ReadingListModel*)model {
   DCHECK(model == _model);

[rohitrao (ping after 24h), 2017/05/09 14:52:08]

Does anyone know why this DCHECK exists or what it's trying to protect/enforce?

[noyau (Ping after 24h), 2017/05/15 14:36:12]

It's enforcing the fact that this method should only be called once. Peter what
do you think about moving the _model=nullptr to the shutdown method instead of
delete?

[pkl (ping after 24h if needed), 2017/05/18 04:51:04]

This should really be here. After rebasing, I noticed that a new
_modelBridge.reset() is added here and I believe that it actually belongs here.
_model should be nulled at the same place as _modelBridge.

   _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