Remove "set wallpaper..." option in system context menu when guest logged in

ash/shell.h

Index: ash/shell.h
diff --git a/ash/shell.h b/ash/shell.h
index 2c0657be4a48ab3b79aaa8e097f893f81c1f7bf3..f7fda3651f74648ab7273efd5d084f4a00785a32 100644
--- a/ash/shell.h
+++ b/ash/shell.h
@@ -72,6 +72,7 @@ class RootWindowEventFilter;
 class RootWindowLayoutManager;
 class ShadowController;
 class ShelfLayoutManager;
+class ShellContextMenu;
 class SystemGestureEventFilter;
 class StackingController;
 class TooltipController;
@@ -291,6 +292,7 @@ class ASH_EXPORT Shell {
 
   scoped_ptr<internal::AppList> app_list_;
 
+  scoped_ptr<internal::ShellContextMenu> shell_context_menu_;
   scoped_ptr<internal::StackingController> stacking_controller_;
   scoped_ptr<internal::ActivationController> activation_controller_;
   scoped_ptr<internal::WindowModalityController> window_modality_controller_;