re-re-land of https://codereview.chromium.org/11364053/

ash/shell.cc

Index: ash/shell.cc
diff --git a/ash/shell.cc b/ash/shell.cc
index e95d1d70243b43d702ff6c97f2872a913e91c735..bac0bb674b5c2d900e78db49791bff4ae4b4b58b 100644
--- a/ash/shell.cc
+++ b/ash/shell.cc
@@ -67,6 +67,7 @@
 #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"
@@ -455,7 +456,9 @@ void Shell::Init() {
   // Create Controllers that may need root window.
   // TODO(oshima): Move as many controllers before creating
   // RootWindowController as possible.
-  stacking_controller_.reset(new internal::StackingController);
+  stacking_client_.reset(delegate_->CreateStackingClient());
+  if (stacking_client_.get())
+    aura::client::SetStackingClient(stacking_client_.get());
   visibility_controller_.reset(new internal::VisibilityController);
   drag_drop_controller_.reset(new internal::DragDropController);
   user_action_client_.reset(delegate_->CreateUserActionClient());
@@ -764,6 +767,10 @@ void Shell::DoInitialWorkspaceAnimation() {
       DoInitialAnimation();
 }
 
+aura::client::StackingClient* Shell::stacking_client() {
+  return stacking_client_.get();
+}
+
 void Shell::InitRootWindowController(
     internal::RootWindowController* controller) {
   aura::RootWindow* root_window = controller->root_window();