Enable WebUI ScreenLocker for aura by abstracting lock window.

chrome/browser/chromeos/login/lock_window.cc

Index: chrome/browser/chromeos/login/lock_window.cc
diff --git a/chrome/browser/chromeos/login/lock_window.cc b/chrome/browser/chromeos/login/lock_window.cc
new file mode 100644
index 0000000000000000000000000000000000000000..fb5265387acbd51a73f4507b9bc5baf39e3a9974
--- /dev/null
+++ b/chrome/browser/chromeos/login/lock_window.cc
@@ -0,0 +1,14 @@
+// Copyright (c) 2011 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 "chrome/browser/chromeos/login/lock_window.h"
+
+#include <cstddef>
+
+namespace chromeos {
+
+LockWindow::LockWindow() : observer_(NULL) {
+}
+
+}  // namespace chromeos