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

ui/aura_shell/stacking_controller.h

Index: ui/aura_shell/stacking_controller.h
diff --git a/ui/aura_shell/stacking_controller.h b/ui/aura_shell/stacking_controller.h
index 2091f1ff072ede6600ba9a59d163c691aa2e84fc..20bb5feaf36531218be6c1c6065586466857d74c 100644
--- a/ui/aura_shell/stacking_controller.h
+++ b/ui/aura_shell/stacking_controller.h
@@ -8,16 +8,22 @@
 
 #include "base/basictypes.h"
 #include "base/compiler_specific.h"
+#include "base/memory/scoped_ptr.h"
 #include "ui/aura/client/stacking_client.h"
 
 namespace aura_shell {
 namespace internal {
 
+class AlwaysOnTopController;
+
 class StackingController : public aura::StackingClient {
  public:
   StackingController();
   virtual ~StackingController();
 
+  // Initializes this controller.
+  void Init();
+
   // Returns true if |window| exists within a container that supports
   // activation.
   static aura::Window* GetActivatableWindow(aura::Window* window);
@@ -29,6 +35,8 @@ class StackingController : public aura::StackingClient {
       aura::Window* ignore) const OVERRIDE;
 
  private:
+  scoped_ptr<internal::AlwaysOnTopController> always_on_top_controller_;
+
   DISALLOW_COPY_AND_ASSIGN(StackingController);
 };