Implement automatic layout and stacking for docked windows

ash/wm/dock/docked_window_layout_manager.h

Index: ash/wm/dock/docked_window_layout_manager.h
diff --git a/ash/wm/dock/docked_window_layout_manager.h b/ash/wm/dock/docked_window_layout_manager.h
index c5ef46959bd72373a66ce5f37e05ab5d9f4497d6..d7e9b0dc49d8137dddca4d53c7cfaac923cd9811 100644
--- a/ash/wm/dock/docked_window_layout_manager.h
+++ b/ash/wm/dock/docked_window_layout_manager.h
@@ -103,6 +103,9 @@ class ASH_EXPORT DockedWindowLayoutManager
   virtual void OnWindowPropertyChanged(aura::Window* window,
                                        const void* key,
                                        intptr_t old) OVERRIDE;
+  virtual void OnWindowBoundsChanged(aura::Window* window,
+                                     const gfx::Rect& old_bounds,
+                                     const gfx::Rect& new_bounds) OVERRIDE;
 
   // aura::client::ActivationChangeObserver:
   virtual void OnWindowActivated(aura::Window* gained_active,
@@ -174,6 +177,10 @@ class ASH_EXPORT DockedWindowLayoutManager
   // Side of the screen that the dock is positioned at.
   DockedAlignment alignment_;
 
+  // The last active window. Used to maintain stacking order even if no windows
+  // are currently focused.
+  aura::Window* last_active_window_;
+
   // Observers of dock bounds changes.
   ObserverList<DockedWindowLayoutManagerObserver> observer_list_;