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

ash/login/ui/lock_screen.cc

 // 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.
 
 #include "ash/login/ui/lock_screen.h"
 
 #include "ash/login/ui/lock_contents_view.h"
 #include "ash/login/ui/lock_window.h"
 #include "ui/display/display.h"
 #include "ui/display/screen.h"
 #include "ui/views/widget/widget.h"
 
 namespace ash {
 
+namespace {
+// Reference to global lock screen instance. There can only ever be one lock
+// screen display at the same time.
+LockWindow* g_window = nullptr;
+}  // namespace
+
 bool ShowLockScreen() {
-  LockWindow* window = new LockWindow();
-  window->SetBounds(display::Screen::GetScreen()->GetPrimaryDisplay().bounds());
+  CHECK(!g_window);
+  g_window = new LockWindow();
+  g_window->SetBounds(
+      display::Screen::GetScreen()->GetPrimaryDisplay().bounds());
 
-  views::View* contents = new LockContentsView();
-  window->SetContentsView(contents);
-  window->Show();
+  auto* contents = new LockContentsView();
+  g_window->SetContentsView(contents);
+  g_window->Show();
 
   return true;
 }
 
+void DestroyLockScreen() {
+  CHECK(g_window);
+  g_window->Close();
+  g_window = nullptr;
+}
+
 }  // namespace ash