make StackingClient dispatch to either ash or desktop

ash/test/test_shell_delegate.cc

Index: ash/test/test_shell_delegate.cc
diff --git a/ash/test/test_shell_delegate.cc b/ash/test/test_shell_delegate.cc
index 8575d6bd71cddbfb55ca1fe4b285dbd9663523c2..c16570794754bab8949898e71a668d74b7cb0032 100644
--- a/ash/test/test_shell_delegate.cc
+++ b/ash/test/test_shell_delegate.cc
@@ -10,6 +10,7 @@
 #include "ash/shell.h"
 #include "ash/shell_window_ids.h"
 #include "ash/test/test_launcher_delegate.h"
+#include "ash/wm/stacking_controller.h"
 #include "ash/wm/window_util.h"
 #include "content/public/test/test_browser_context.h"
 #include "ui/aura/window.h"
@@ -159,5 +160,9 @@ double TestShellDelegate::GetSavedScreenMagnifierScale() {
   return std::numeric_limits<double>::min();
 }
 
+aura::client::StackingClient* TestShellDelegate::CreateStackingClient() {
+  return new StackingController;
+}
+
 }  // namespace test
 }  // namespace ash