[Aura] Support always-on-top top level window.

ui/aura_shell/always_on_top_controller.h

Index: ui/aura_shell/always_on_top_controller.h
diff --git a/ui/aura_shell/always_on_top_controller.h b/ui/aura_shell/always_on_top_controller.h
new file mode 100644
index 0000000000000000000000000000000000000000..e0690b391d653f8dc726852a0c1585fafa1a07c3
--- /dev/null
+++ b/ui/aura_shell/always_on_top_controller.h
@@ -0,0 +1,54 @@
+// Copyright (c) 2011 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.
+
+#ifndef UI_AURA_SHELL_ALWAYS_ON_TOP_CONTROLLER_H_
+#define UI_AURA_SHELL_ALWAYS_ON_TOP_CONTROLLER_H_
+#pragma once
+
+#include "base/basictypes.h"
+#include "base/compiler_specific.h"
+#include "ui/aura/window_observer.h"
+
+namespace aura {
+class Window;
+}
+
+namespace aura_shell {
+namespace internal {
+
+// AlwaysOnTopController puts window into proper containers based on its
+// 'AlwaysOnTop' property. That is, putting a window to |default_container_|
+// if its "AlwaysOnTop" property is false. Otherwise, put it in
+// |always_on_top_container_|.
+class AlwaysOnTopController : public aura::WindowObserver {
+ public:
+  AlwaysOnTopController();
+  virtual ~AlwaysOnTopController();
+
+  // Sets the two top level window containers.
+  void SetContainers(aura::Window* default_container,
+                     aura::Window* always_on_top_container);
+
+  // Gets container for given |window| based on its "AlwaysOnTop" property.
+  aura::Window* GetContainer(aura::Window* window) const;
+
+ private:
+  // Overridden from aura::WindowObserver:
+  virtual void OnWindowAdded(aura::Window* child) OVERRIDE;
+  virtual void OnWillRemoveWindow(aura::Window* child) OVERRIDE;
+  virtual void OnPropertyChanged(aura::Window* window,
+                                 const char* name,
+                                 void* old) OVERRIDE;
+  virtual void OnWindowDestroyed(aura::Window* window) OVERRIDE;
+
+  aura::Window* default_container_;
+  aura::Window* always_on_top_container_;
+
+  DISALLOW_COPY_AND_ASSIGN(AlwaysOnTopController);
+};
+
+}  // namepsace aura_shell
+}  // namepsace internal
+
+#endif  // UI_AURA_SHELL_ALWAYS_ON_TOP_CONTROLLER_H_