cros: Make sure views-based lock screen is destroyed after it is dismissed.

ash/login/ui/lock_screen.h

Index: ash/login/ui/lock_screen.h
diff --git a/ash/login/ui/lock_screen.h b/ash/login/ui/lock_screen.h
index fcccca615ae29945b5f6c25c0a8ecbde3d25fc2a..c22164770201b0da40d65c2cffeefe89952b6a6b 100644
--- a/ash/login/ui/lock_screen.h
+++ b/ash/login/ui/lock_screen.h
@@ -15,6 +15,9 @@ namespace ash {
 // The lock screen communicates with the backend C++ via a mojo API.
 ASH_EXPORT bool ShowLockScreen();
 
+// Destroys the lock screen. There must be an existing lock screen instance.
+ASH_EXPORT void DestroyLockScreen();
+
 }  // namespace ash
 
 #endif  // ASH_LOGIN_UI_LOCK_SCREEN_H_