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

chrome/browser/ui/ash/chrome_shell_delegate.cc

Index: chrome/browser/ui/ash/chrome_shell_delegate.cc
diff --git a/chrome/browser/ui/ash/chrome_shell_delegate.cc b/chrome/browser/ui/ash/chrome_shell_delegate.cc
index 624b2c8239d09a61fc592076e29156b080f087b0..6e2975a960b60f997c167e1b00d59c8b5acaec23 100644
--- a/chrome/browser/ui/ash/chrome_shell_delegate.cc
+++ b/chrome/browser/ui/ash/chrome_shell_delegate.cc
@@ -6,6 +6,7 @@
 
 #include "ash/launcher/launcher_types.h"
 #include "ash/system/tray/system_tray_delegate.h"
+#include "ash/wm/stacking_controller.h"
 #include "ash/wm/window_util.h"
 #include "base/bind.h"
 #include "base/command_line.h"
@@ -471,6 +472,10 @@ ui::MenuModel* ChromeShellDelegate::CreateContextMenu(aura::RootWindow* root) {
   return new LauncherContextMenu(launcher_delegate_, root);
 }
 
+aura::client::StackingClient* ChromeShellDelegate::CreateStackingClient() {
+  return new ash::StackingController;
+}
+
 void ChromeShellDelegate::Observe(int type,
                                   const content::NotificationSource& source,
                                   const content::NotificationDetails& details) {