Use MD-ash's auto hide behavior for arc++ windows.

chrome/browser/chromeos/login/lock/screen_locker.cc

Index: chrome/browser/chromeos/login/lock/screen_locker.cc
diff --git a/chrome/browser/chromeos/login/lock/screen_locker.cc b/chrome/browser/chromeos/login/lock/screen_locker.cc
index bffe739b0a78a3888c2b5780634e16f315b77887..7c99b746ed5f5f0962fec6da1865fe575841280a 100644
--- a/chrome/browser/chromeos/login/lock/screen_locker.cc
+++ b/chrome/browser/chromeos/login/lock/screen_locker.cc
@@ -423,9 +423,9 @@ void ScreenLocker::Show() {
   // visible while in fullscreen because the shelf makes it harder for a web
   // page or app to mimick the lock screen.
   ash::wm::WindowState* active_window_state = ash::wm::GetActiveWindowState();
-  if (active_window_state &&
-      active_window_state->IsFullscreen() &&
-      active_window_state->hide_shelf_when_fullscreen()) {
+  if (active_window_state && active_window_state->IsFullscreen() &&
+      active_window_state->shelf_mode_in_fullscreen() !=
+          ash::wm::WindowState::SHELF_AUTO_HIDE_VISIBLE) {
     const ash::wm::WMEvent event(ash::wm::WM_EVENT_TOGGLE_FULLSCREEN);
     active_window_state->OnWMEvent(&event);
   }