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

ui/aura_shell/stacking_controller_unittest.cc

Index: ui/aura_shell/stacking_controller_unittest.cc
diff --git a/ui/aura_shell/stacking_controller_unittest.cc b/ui/aura_shell/stacking_controller_unittest.cc
index a7cd2c9ee2d3461c927ab9aec972c3511f614348..4c57c2489b431245b950b06afbf950403e2e4d6c 100644
--- a/ui/aura_shell/stacking_controller_unittest.cc
+++ b/ui/aura_shell/stacking_controller_unittest.cc
@@ -17,6 +17,7 @@ typedef aura::test::AuraTestBase StackingControllerTest;
 
 TEST_F(StackingControllerTest, GetTopmostWindowToActivate) {
   Shell::GetInstance();
+
   aura::test::ActivateWindowDelegate activate;
   aura::test::ActivateWindowDelegate non_activate(false);
 
@@ -28,6 +29,9 @@ TEST_F(StackingControllerTest, GetTopmostWindowToActivate) {
       &non_activate, 3, gfx::Rect(), NULL));
   EXPECT_EQ(w2.get(), aura::Desktop::GetInstance()->stacking_client()->
       GetTopmostWindowToActivate(NULL));
+
+  FlushMessageLoop();
+  Shell::DeleteInstanceForTesting();
 }
 
 }  // namespace test