[Aura] Support always-on-top top level window.

ui/aura_shell/stacking_controller_unittest.cc

 // 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 "ui/aura_shell/stacking_controller.h"
 
 #include "ui/aura/desktop.h"
 #include "ui/aura/test/aura_test_base.h"
 #include "ui/aura/test/test_windows.h"
 #include "ui/aura/test/test_window_delegate.h"
 #include "ui/aura_shell/shell.h"
 
 namespace aura_shell {
 namespace test {
 
 typedef aura::test::AuraTestBase StackingControllerTest;
 
 TEST_F(StackingControllerTest, GetTopmostWindowToActivate) {
   Shell::GetInstance();
+
   aura::test::ActivateWindowDelegate activate;
   aura::test::ActivateWindowDelegate non_activate(false);
 
   scoped_ptr<aura::Window> w1(aura::test::CreateTestWindowWithDelegate(
       &non_activate, 1, gfx::Rect(), NULL));
   scoped_ptr<aura::Window> w2(aura::test::CreateTestWindowWithDelegate(
       &activate, 2, gfx::Rect(), NULL));
   scoped_ptr<aura::Window> w3(aura::test::CreateTestWindowWithDelegate(
       &non_activate, 3, gfx::Rect(), NULL));
   EXPECT_EQ(w2.get(), aura::Desktop::GetInstance()->stacking_client()->
       GetTopmostWindowToActivate(NULL));
+
+  FlushMessageLoop();
+  Shell::DeleteInstanceForTesting();
 }
 
 }  // namespace test
 }  // namespace aura_shell