make StackingClient dispatch to either ash or desktop

ash/shell.cc

Index: ash/shell.cc
diff --git a/ash/shell.cc b/ash/shell.cc
index 97f3c8f1bffe20c086215429e7ee2f555df9f701..f6c0ac0db0633437e4423fec038a561135af3878 100644
--- a/ash/shell.cc
+++ b/ash/shell.cc
@@ -66,6 +66,7 @@
 #include "base/command_line.h"
 #include "base/debug/leak_annotations.h"
 #include "ui/aura/client/aura_constants.h"
+#include "ui/aura/client/stacking_client.h"
 #include "ui/aura/client/user_action_client.h"
 #include "ui/aura/display_manager.h"
 #include "ui/aura/env.h"
@@ -451,6 +452,9 @@ void Shell::Init() {
   // TODO(oshima): Move as many controllers before creating
   // RootWindowController as possible.
   stacking_controller_.reset(new internal::StackingController);
+#if !defined(OS_WIN)
+  aura::client::SetStackingController(stacking_controller_.get());
+#endif
   visibility_controller_.reset(new internal::VisibilityController);
   drag_drop_controller_.reset(new internal::DragDropController);
   user_action_client_.reset(delegate_->CreateUserActionClient());
@@ -763,6 +767,10 @@ void Shell::DoInitialWorkspaceAnimation() {
       DoInitialAnimation();
 }
 
+aura::client::StackingClient* Shell::stacking_client() {
+  return stacking_controller_.get();
+}
+
 void Shell::InitRootWindowController(
     internal::RootWindowController* controller) {
   aura::RootWindow* root_window = controller->root_window();