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

ui/aura_shell/shell.cc

 // 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.
 
 #include "ui/aura_shell/shell.h"
 
 #include "base/bind.h"
 #include "ui/aura/desktop.h"
 #include "ui/aura/screen_aura.h"
 #include "ui/aura/toplevel_window_container.h"
@@ skipping 145 matching lines @@
 void Shell::AddChildToDefaultParent(aura::Window* window) {
   aura::Window* parent = NULL;
   intptr_t type = reinterpret_cast<intptr_t>(
       ui::ViewProp::GetValue(window, views::NativeWidgetAura::kWindowTypeKey));
   switch (static_cast<Widget::InitParams::Type>(type)) {
     case Widget::InitParams::TYPE_WINDOW:
     case Widget::InitParams::TYPE_WINDOW_FRAMELESS:
     case Widget::InitParams::TYPE_CONTROL:
     case Widget::InitParams::TYPE_BUBBLE:
     case Widget::InitParams::TYPE_POPUP:
-      parent = GetContainer(internal::kShellWindowId_DefaultContainer);
+      if (!!window->GetProperty(aura::Window::kPropAlwaysOnTop))
+        parent = GetContainer(internal::kShellWindowId_AlwaysOnTopContainer);
+      else
+        parent = GetContainer(internal::kShellWindowId_DefaultContainer);
       break;
     case Widget::InitParams::TYPE_MENU:
     case Widget::InitParams::TYPE_TOOLTIP:
       parent = GetContainer(internal::kShellWindowId_MenusAndTooltipsContainer);
       break;
     default:
       // This will crash for controls, since they can't be parented to anything.
       break;
   }
   parent->AddChild(window);
 }
 
 aura::Window* Shell::GetTopmostWindowToActivate(aura::Window* ignore) const {
   const aura::ToplevelWindowContainer* container =
       GetContainer(internal::kShellWindowId_DefaultContainer)->
           AsToplevelWindowContainer();
   return container->GetTopmostWindowToActivate(ignore);
 }
 
 }  // namespace aura_shell