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

ui/app_list/views/apps_grid_view_delegate.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_APPS_GRID_VIEW_DELEGATE_H_
 #define UI_APP_LIST_VIEWS_APPS_GRID_VIEW_DELEGATE_H_
 
 #include <string>
 
 #include "base/callback_forward.h"
@@ skipping 12 matching lines @@
   // Invoked when an item is activated on the grid view. |event_flags| contains
   // the flags of the keyboard/mouse event that triggers the activation request.
   virtual void ActivateApp(AppListItem* item, int event_flags) = 0;
 
   // Gets the path to a shortcut for the app represented by |item|. |callback|
   // may be run immediately.
   virtual void GetShortcutPathForApp(
       const std::string& app_id,
       const base::Callback<void(const base::FilePath&)>& callback) = 0;
 
+  // Called by the root grid view to cancel a drag that started inside a folder.
+  // This can occur when the root grid is visible for a reparent and its model
+  // changes, necessitating a cancel of the drag operation.
+  virtual void CancelDragInActiveFolder() = 0;
+
  protected:
   virtual ~AppsGridViewDelegate() {}
 };
 
 }  // namespace app_list
 
 #endif  // UI_APP_LIST_VIEWS_APPS_GRID_VIEW_DELEGATE_H_