Protect AppListItemList Add/Remove and fix sync bugs

ui/app_list/app_list_model_observer.h

Index: ui/app_list/app_list_model_observer.h
diff --git a/ui/app_list/app_list_model_observer.h b/ui/app_list/app_list_model_observer.h
index 8810c821c564ffe1d04398a01ff1db3f41668385..cc3d08e185309c83ebcf2728b7275c0fcc354f78 100644
--- a/ui/app_list/app_list_model_observer.h
+++ b/ui/app_list/app_list_model_observer.h
@@ -9,11 +9,22 @@
 
 namespace app_list {
 
+class AppListItem;
+
 class APP_LIST_EXPORT AppListModelObserver {
  public:
   // Invoked when AppListModel's status has changed.
   virtual void OnAppListModelStatusChanged() {}
 
+  // Triggered after |item| has been added to the model.
+  virtual void OnAppListItemAdded(AppListItem* item) {}
+
+  // Triggered just before an item is deleted from the model.
+  virtual void OnAppListItemWillBeDeleted(AppListItem* item) {}
+
+  // Triggered after |item| has moved or changed folders.
+  virtual void OnAppListItemUpdated(AppListItem* item) {}
+
  protected:
   virtual ~AppListModelObserver() {}
 };