[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 "base/command_line.h"
+#include "ui/aura/aura_constants.h"
 #include "ui/aura/aura_switches.h"
 #include "ui/aura/desktop.h"
 #include "ui/aura/screen_aura.h"
 #include "ui/aura/toplevel_window_container.h"
 #include "ui/aura/window.h"
 #include "ui/aura/window_types.h"
 #include "ui/aura_shell/default_container_event_filter.h"
 #include "ui/aura_shell/default_container_layout_manager.h"
 #include "ui/aura_shell/desktop_layout_manager.h"
 #include "ui/aura_shell/launcher/launcher.h"
@@ skipping 56 matching lines @@
 
 ////////////////////////////////////////////////////////////////////////////////
 // Shell, public:
 
 Shell::Shell()
     : ALLOW_THIS_IN_INITIALIZER_LIST(method_factory_(this)) {
   aura::Desktop::GetInstance()->SetDelegate(this);
 }
 
 Shell::~Shell() {
+  DCHECK(instance_ == this);
+  instance_ = NULL;
 }
 
 // static
 Shell* Shell::GetInstance() {
   if (!instance_) {
     instance_ = new Shell;
     instance_->Init();
   }
   return instance_;
 }
@@ skipping 70 matching lines @@
 }
 
 ////////////////////////////////////////////////////////////////////////////////
 // Shell, aura::DesktopDelegate implementation:
 
 void Shell::AddChildToDefaultParent(aura::Window* window) {
   aura::Window* parent = NULL;
   switch (window->type()) {
     case aura::WINDOW_TYPE_NORMAL:
     case aura::WINDOW_TYPE_POPUP:
-      parent = GetContainer(internal::kShellWindowId_DefaultContainer);
+      if (!!window->GetProperty(aura::kAlwaysOnTopKey))
+        parent = GetContainer(internal::kShellWindowId_AlwaysOnTopContainer);
+      else
+        parent = GetContainer(internal::kShellWindowId_DefaultContainer);
       break;
     case aura::WINDOW_TYPE_MENU:
     case aura::WINDOW_TYPE_TOOLTIP:
       parent = GetContainer(internal::kShellWindowId_MenusAndTooltipsContainer);
       break;
     default:
       NOTREACHED() << "Window " << window->id()
                    << " has unhandled type " << window->type();
       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