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

ash/login/ui/lock_screen.h

 // Copyright 2017 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef ASH_LOGIN_UI_LOCK_SCREEN_H_
 #define ASH_LOGIN_UI_LOCK_SCREEN_H_
 
 #include "ash/ash_export.h"
 
 namespace ash {
 
+// TODO(jdufault): There's some internal state here so put ShowLockScreen and
+// DestroyLockScreen inside a (static) class, ie, ash::LockScreen::Show() and
+// ash::LockScreen::Destroy().
+
 // Creates and displays the lock screen. Returns true if the lock screen was
 // displayed.
 //
 // 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_