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

ash/login/ui/lock_screen.cc

Index: ash/login/ui/lock_screen.cc
diff --git a/ash/login/ui/lock_screen.cc b/ash/login/ui/lock_screen.cc
index baaf5bfd80b602c59fc51ead9cfa6aae0027c8f9..d13311deb0173968ca734781c9425b1768000020 100644
--- a/ash/login/ui/lock_screen.cc
+++ b/ash/login/ui/lock_screen.cc
@@ -12,15 +12,29 @@
 
 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