Fix app list DCHECKs getting hit when model updates occur during dragging

ui/app_list/views/app_list_main_view.h

 // Copyright 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.
 
 #ifndef UI_APP_LIST_VIEWS_APP_LIST_MAIN_VIEW_H_
 #define UI_APP_LIST_VIEWS_APP_LIST_MAIN_VIEW_H_
 
 #include <string>
 
 #include "base/memory/scoped_vector.h"
@@ skipping 83 matching lines @@
   void OnIconLoadingWaitTimer();
 
   // Invoked from an IconLoader when icon loading is finished.
   void OnItemIconLoaded(IconLoader* loader);
 
   // Overridden from AppsGridViewDelegate:
   virtual void ActivateApp(AppListItem* item, int event_flags) OVERRIDE;
   virtual void GetShortcutPathForApp(
       const std::string& app_id,
       const base::Callback<void(const base::FilePath&)>& callback) OVERRIDE;
+  virtual void CancelDragInActiveFolder() OVERRIDE;
 
   // Overridden from SearchBoxViewDelegate:
   virtual void QueryChanged(SearchBoxView* sender) OVERRIDE;
 
   // Overridden from SearchResultListViewDelegate:
   virtual void OnResultInstalled(SearchResult* result) OVERRIDE;
   virtual void OnResultUninstalled(SearchResult* result) OVERRIDE;
 
   AppListViewDelegate* delegate_;  // Owned by parent view (AppListView).
   AppListModel* model_;  // Unowned; ownership is handled by |delegate_|.
@@ skipping 11 matching lines @@
   ScopedVector<IconLoader> pending_icon_loaders_;
 
   base::WeakPtrFactory<AppListMainView> weak_ptr_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(AppListMainView);
 };
 
 }  // namespace app_list
 
 #endif  // UI_APP_LIST_VIEWS_APP_LIST_MAIN_VIEW_H_