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

chrome/browser/ui/aura/stacking_client_aura.cc

Index: chrome/browser/ui/aura/stacking_client_aura.cc
diff --git a/chrome/browser/ui/aura/stacking_client_aura.cc b/chrome/browser/ui/aura/stacking_client_aura.cc
new file mode 100644
index 0000000000000000000000000000000000000000..569c7a5884d089918323061c35e9f6e56db10ff5
--- /dev/null
+++ b/chrome/browser/ui/aura/stacking_client_aura.cc
@@ -0,0 +1,33 @@
+// Copyright (c) 2012 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "chrome/browser/ui/aura/stacking_client_aura.h"
+
+#include "ash/shell.h"
+#include "ash/wm/stacking_controller.h"
+#include "ui/aura/desktop/desktop_stacking_client.h"
+#include "ui/aura/focus_manager.h"
+#include "ui/aura/root_window.h"
+#include "ui/aura/window.h"
+#include "ui/views/widget/desktop_native_widget_aura.h"
+
+StackingClientAura::StackingClientAura() {
+  desktop_stacking_client_.reset(new aura::DesktopStackingClient);
+}
+
+StackingClientAura::~StackingClientAura() {
+}
+
+aura::Window* StackingClientAura::GetDefaultParent(aura::Window* context,
+                                                   aura::Window* window,
+                                                   const gfx::Rect& bounds) {
+#if defined(USE_ASH)
+  if (chrome::GetHostDesktopTypeForNativeView(context) ==
+      chrome::HOST_DESKTOP_TYPE_ASH) {
+    return ash::Shell::GetInstance()->stacking_client()->GetDefaultParent(
+        context, window, bounds);
+  }
+#endif
+  return desktop_stacking_client_->GetDefaultParent(context, window, bounds);
+}