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

ash/wm/stacking_controller.cc

Index: ash/wm/stacking_controller.cc
diff --git a/ash/wm/stacking_controller.cc b/ash/wm/stacking_controller.cc
index f5b2c065c56639c0af89051b21fc74531893ec1a..a5517d13ea403cd8a061dbca2bb25079f6232ffc 100644
--- a/ash/wm/stacking_controller.cc
+++ b/ash/wm/stacking_controller.cc
@@ -17,7 +17,6 @@
 #include "ui/base/ui_base_types.h"
 
 namespace ash {
-namespace internal {
 namespace {
 
 // Find a root window that matches the |bounds|. If the virtual screen
@@ -56,7 +55,6 @@ bool IsWindowModal(aura::Window* window) {
 // StackingController, public:
 
 StackingController::StackingController() {
-  aura::client::SetStackingClient(this);
 }
 
 StackingController::~StackingController() {
@@ -65,7 +63,8 @@ StackingController::~StackingController() {
 ////////////////////////////////////////////////////////////////////////////////
 // StackingController, aura::StackingClient implementation:
 
-aura::Window* StackingController::GetDefaultParent(aura::Window* window,
+aura::Window* StackingController::GetDefaultParent(aura::Window* context,
+                                                   aura::Window* window,
                                                    const gfx::Rect& bounds) {
   aura::RootWindow* target_root = NULL;
   if (window->transient_parent()) {
@@ -148,10 +147,9 @@ StackingController::GetAlwaysOnTopController(aura::RootWindow* root_window) {
         root_window->GetChildById(
             internal::kShellWindowId_AlwaysOnTopContainer));
     // RootWindow owns the AlwaysOnTopController object.
-    root_window->SetProperty(kAlwaysOnTopControllerKey, controller);
+    root_window->SetProperty(internal::kAlwaysOnTopControllerKey, controller);
   }
   return controller;
 }
 
-}  // namespace internal
 }  // namespace ash