Add always on top windows to the alt+tab list

ash/shell.cc

Index: ash/shell.cc
diff --git a/ash/shell.cc b/ash/shell.cc
index 223438a3b710f9e38c0019c893c0813d2326ad4d..3a3c31c7b24725826646dc05e7fd045321055daa 100644
--- a/ash/shell.cc
+++ b/ash/shell.cc
@@ -701,6 +701,7 @@ void Shell::InitRootWindowController(
   DCHECK(visibility_controller_.get());
   DCHECK(drag_drop_controller_.get());
   DCHECK(capture_controller_.get());
+  DCHECK(window_cycle_controller_.get());
 
   root_window->set_focus_manager(focus_manager_.get());
   input_method_filter_->SetInputMethodPropertyInRootWindow(root_window);
@@ -730,6 +731,8 @@ void Shell::InitRootWindowController(
       root_window->GetChildById(internal::kShellWindowId_AlwaysOnTopContainer));
   root_window->SetProperty(internal::kAlwaysOnTopControllerKey,
                            always_on_top_controller);
+
+  window_cycle_controller_->OnRootWindowAdded(root_window);
 }
 
 ////////////////////////////////////////////////////////////////////////////////