Store AppItems as pages in AppListModel.

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 e409fa7fb436efb211624fd35bf5a093430f9f85..bd6d40fb357987974202663d2bd8507fd858329d 100644
--- a/ui/app_list/app_list_model_observer.h
+++ b/ui/app_list/app_list_model_observer.h
@@ -20,6 +20,14 @@ class APP_LIST_EXPORT AppListModelObserver {
   // Invoked when AppListModel's current user's signin status has changed.
   virtual void OnAppListModelSigninStatusChanged() {}
 
+  // Invoked when items on |page_index| change.
+  virtual void OnListItemsAdded(size_t page_index,
+                                size_t start, size_t count) {}
+  virtual void OnListItemsRemoved(size_t page_index,
+                                  size_t start, size_t count) {}
+  virtual void OnListItemMoved(size_t page_index,
+                               size_t index, size_t target_index) {}
+
  protected:
   virtual ~AppListModelObserver() {}
 };