[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 705e0ac1d05dda05822de7bf9a8fce4be8742ef5..56a5339de80f2bdb36c883327f0bac61450efaea 100644
--- a/ui/aura_shell/shell.cc
+++ b/ui/aura_shell/shell.cc
@@ -6,6 +6,7 @@
 
 #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"
@@ -82,6 +83,8 @@ Shell::Shell()
 }
 
 Shell::~Shell() {
+  DCHECK(instance_ == this);
+  instance_ = NULL;
 }
 
 // static
@@ -172,7 +175,10 @@ void Shell::AddChildToDefaultParent(aura::Window* window) {
   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: