Second attempt at moving the StackingClient to a property on the RootWindow.

ui/aura_shell/shell.h

Index: ui/aura_shell/shell.h
===================================================================
--- ui/aura_shell/shell.h	(revision 114527)
+++ ui/aura_shell/shell.h	(working copy)
@@ -38,6 +38,7 @@
 class DragDropController;
 class ShadowController;
 class ShellAcceleratorFilter;
+class StackingController;
 class WorkspaceController;
 }
 
@@ -117,6 +118,7 @@
 
   scoped_ptr<internal::AppList> app_list_;
 
+  scoped_ptr<internal::StackingController> stacking_controller_;
   scoped_ptr<internal::ActivationController> activation_controller_;
   scoped_ptr<internal::DragDropController> drag_drop_controller_;
   scoped_ptr<internal::WorkspaceController> workspace_controller_;