chromeos: Remove some WmWindow usage from ash/system

ash/system/tray/system_tray_controller.cc

Index: ash/system/tray/system_tray_controller.cc
diff --git a/ash/system/tray/system_tray_controller.cc b/ash/system/tray/system_tray_controller.cc
index 5f3d07a08c30ca5d03089df12ab6e0671ea72e11..c2ba4b72bcaea78c05b316bedb3a6ddf5825420e 100644
--- a/ash/system/tray/system_tray_controller.cc
+++ b/ash/system/tray/system_tray_controller.cc
@@ -200,8 +200,8 @@ void SystemTrayController::ShowUpdateIcon(mojom::UpdateSeverity severity,
                                           bool factory_reset_required,
                                           mojom::UpdateType update_type) {
   // Show the icon on all displays.
-  for (WmWindow* root : ShellPort::Get()->GetAllRootWindows()) {
-    ash::SystemTray* tray = root->GetRootWindowController()->GetSystemTray();
+  for (RootWindowController* root : Shell::GetAllRootWindowControllers()) {
+    ash::SystemTray* tray = root->GetSystemTray();
     // External monitors might not have a tray yet.
     if (!tray)
       continue;