Fix the content_browsertests everywhere maybe.

ash/root_window_controller.cc

Index: ash/root_window_controller.cc
diff --git a/ash/root_window_controller.cc b/ash/root_window_controller.cc
index 543a0840bc8a2961d5ab8f181c2244938173b822..9c6580a253a7fbf3387628817d3d32a7a646fc50 100644
--- a/ash/root_window_controller.cc
+++ b/ash/root_window_controller.cc
@@ -26,6 +26,7 @@
 #include "ash/wm/root_window_layout_manager.h"
 #include "ash/wm/screen_dimmer.h"
 #include "ash/wm/shelf_layout_manager.h"
+#include "ash/wm/stacking_controller.h"
 #include "ash/wm/status_area_layout_manager.h"
 #include "ash/wm/system_background_controller.h"
 #include "ash/wm/system_modal_container_layout_manager.h"
@@ -168,6 +169,9 @@ RootWindowController::RootWindowController(aura::RootWindow* root_window)
       panel_layout_manager_(NULL) {
   SetRootWindowController(root_window, this);
   screen_dimmer_.reset(new ScreenDimmer(root_window));
+
+  stacking_controller_.reset(new ash::StackingController);
+  aura::client::SetStackingClient(root_window, stacking_controller_.get());
 }
 
 RootWindowController::~RootWindowController() {