Notify launcher page with onTransitionChanged event

ui/app_list/pagination_model_observer.h

Index: ui/app_list/pagination_model_observer.h
diff --git a/ui/app_list/pagination_model_observer.h b/ui/app_list/pagination_model_observer.h
index ee594eca8938d29d06fa839e7094926dfe7c8787..e3338b70866a5e39d7e4432b2b414089d5ef7cfc 100644
--- a/ui/app_list/pagination_model_observer.h
+++ b/ui/app_list/pagination_model_observer.h
@@ -12,16 +12,16 @@ 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() {}