mash: Cleanup ash shelf application menu code.

ash/shell/window_watcher_shelf_item_delegate.cc

Index: ash/shell/window_watcher_shelf_item_delegate.cc
diff --git a/ash/shell/window_watcher_shelf_item_delegate.cc b/ash/shell/window_watcher_shelf_item_delegate.cc
index cfceb3c32b56b46eb32805162112861120812e86..935186d56a7dd45ab0da9557192eca83384b2f54 100644
--- a/ash/shell/window_watcher_shelf_item_delegate.cc
+++ b/ash/shell/window_watcher_shelf_item_delegate.cc
@@ -31,9 +31,10 @@ ShelfItemDelegate::PerformedAction WindowWatcherShelfItemDelegate::ItemSelected(
   return kExistingWindowActivated;
 }
 
-ui::SimpleMenuModel* WindowWatcherShelfItemDelegate::CreateApplicationMenu(
+ShelfAppMenuItemList WindowWatcherShelfItemDelegate::GetAppMenuItems(
     int event_flags) {
-  return nullptr;
+  // Return an empty item list to avoid showing an application menu.
+  return ShelfAppMenuItemList();
 }
 
 void WindowWatcherShelfItemDelegate::Close() {}