[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 500c24fd32b7efb0fe59953d82c06d849556bd24..cf4fdc7c3a4959180b38a8f865ea94bde78a4cc5 100644
--- a/ui/aura_shell/stacking_controller_unittest.cc
+++ b/ui/aura_shell/stacking_controller_unittest.cc
@@ -5,18 +5,16 @@
 #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"
+#include "ui/aura_shell/test/aura_shell_test_base.h"
 
 namespace aura_shell {
 namespace test {
 
-typedef aura::test::AuraTestBase StackingControllerTest;
+typedef aura_shell::test::AuraShellTestBase StackingControllerTest;
 
 TEST_F(StackingControllerTest, GetTopmostWindowToActivate) {
-  Shell::CreateInstance(NULL);
   aura::test::ActivateWindowDelegate activate;
   aura::test::ActivateWindowDelegate non_activate(false);