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

ui/aura_shell/shell.cc

Index: ui/aura_shell/shell.cc
diff --git a/ui/aura_shell/shell.cc b/ui/aura_shell/shell.cc
index b2a3a98651b4f957f69c7871265161626fa26ff9..f540ac2117da89b75852d0de43c483deb3e6b7ca 100644
--- a/ui/aura_shell/shell.cc
+++ b/ui/aura_shell/shell.cc
@@ -11,6 +11,7 @@
 #include "ui/aura/toplevel_window_container.h"
 #include "ui/aura/window.h"
 #include "ui/aura/window_types.h"
+#include "ui/aura_shell/always_on_top_controller.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"
@@ -82,6 +83,8 @@ Shell::Shell()
 }
 
 Shell::~Shell() {
+  DCHECK(instance_ == this);
+  instance_ = NULL;
 }
 
 // static
@@ -106,6 +109,11 @@ void Shell::Init() {
     (*i)->Show();
   }
 
+  always_on_top_controller_.reset(new internal::AlwaysOnTopController);
+  always_on_top_controller_->SetContainers(
+    GetContainer(internal::kShellWindowId_DefaultContainer),
+    GetContainer(internal::kShellWindowId_AlwaysOnTopContainer));
+
   internal::DesktopLayoutManager* desktop_layout =
       new internal::DesktopLayoutManager(desktop_window);
   desktop_window->SetLayoutManager(desktop_layout);
@@ -175,7 +183,7 @@ void Shell::AddChildToDefaultParent(aura::Window* window) {
   switch (window->type()) {
     case aura::WINDOW_TYPE_NORMAL:
     case aura::WINDOW_TYPE_POPUP:
-      parent = GetContainer(internal::kShellWindowId_DefaultContainer);
+      parent = always_on_top_controller_->GetContainer(window);
       break;
     case aura::WINDOW_TYPE_MENU:
     case aura::WINDOW_TYPE_TOOLTIP: