Fix Mac App Launcher observer lifetime on AppListItemList

ui/app_list/app_list_item_list.cc

 // Copyright (c) 2013 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.
 
 #include "ui/app_list/app_list_item_list.h"
 
 #include "ui/app_list/app_list_item.h"
 
 namespace app_list {
 
 AppListItemList::AppListItemList() {
 }
 
 AppListItemList::~AppListItemList() {
 }
 
 void AppListItemList::AddObserver(AppListItemListObserver* observer) {
   observers_.AddObserver(observer);
 }
 
 void AppListItemList::RemoveObserver(AppListItemListObserver* observer) {
+  DCHECK(observers_.HasObserver(observer));
   observers_.RemoveObserver(observer);
 }
 
 AppListItem* AppListItemList::FindItem(const std::string& id) {
   for (size_t i = 0; i < app_list_items_.size(); ++i) {
     AppListItem* item = app_list_items_[i];
     if (item->id() == id)
       return item;
   }
   return NULL;
@@ skipping 204 matching lines @@
     else
       cur->set_position(prev->position().CreateAfter());
     prev = cur;
   }
   FOR_EACH_OBSERVER(AppListItemListObserver,
                     observers_,
                     OnListItemMoved(index, index, app_list_items_[index]));
 }
 
 }  // namespace app_list