reland of https://codereview.chromium.org/11364053/ w/o removing the per-rootwindow stacking client

ash/shell.cc

Index: ash/shell.cc
diff --git a/ash/shell.cc b/ash/shell.cc
index 579bd765c95e1ac3e6958f7a4b1290623e731723..3a942949a6c792195821830ae1815e5a892c19b9 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"
@@ -454,7 +455,9 @@ 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());
@@ -771,6 +774,10 @@ 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();