Notify launcher page with onTransitionChanged event

ui/app_list/pagination_model_observer.h

 // Copyright (c) 2012 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_PAGINATION_MODEL_OBSERVER_H_
 #define UI_APP_LIST_PAGINATION_MODEL_OBSERVER_H_
 
 #include "ui/app_list/app_list_export.h"
 
 namespace app_list {
 
 class APP_LIST_EXPORT PaginationModelObserver {
  public:
   // Invoked when the total number of page is changed.
-  virtual void TotalPagesChanged() = 0;
+  virtual void TotalPagesChanged() {}
 
   // Invoked when the selected page index is changed.
-  virtual void SelectedPageChanged(int old_selected, int new_selected) = 0;
+  virtual void SelectedPageChanged(int old_selected, int new_selected) {}
 
   // Invoked when a transition starts.
-  virtual void TransitionStarted() = 0;
+  virtual void TransitionStarted() {}
 
   // Invoked when the transition data is changed.
-  virtual void TransitionChanged() = 0;
+  virtual void TransitionChanged() {}
 
  protected:
   virtual ~PaginationModelObserver() {}
 };
 
 }  // namespace app_list
 
 #endif  // UI_APP_LIST_PAGINATION_MODEL_OBSERVER_H_