make StackingClient dispatch to either ash or desktop

ash/shell.h

Index: ash/shell.h
diff --git a/ash/shell.h b/ash/shell.h
index 41ce0dd2019367fc0f6d46e690fb19a4a1032177..98318bb6e07f1d88d95d838981a8b71a356b4044 100644
--- a/ash/shell.h
+++ b/ash/shell.h
@@ -31,6 +31,7 @@ class FocusManager;
 class RootWindow;
 class Window;
 namespace client {
+class StackingClient;
 class UserActionClient;
 }
 namespace shared {
@@ -410,6 +411,8 @@ class ASH_EXPORT Shell : internal::SystemModalContainerEventFilterDelegate,
   }
 #endif  // defined(OS_CHROMEOS)
 
+ aura::client::StackingClient* stacking_client();
+
  private:
   FRIEND_TEST_ALL_PREFIXES(ExtendedDesktopTest, TestCursor);
   FRIEND_TEST_ALL_PREFIXES(WindowManagerTest, MouseEventCursors);