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..390dd0b353171ea4712f8f6f4fe8cd6639044bcd 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,
@@ -156,6 +159,8 @@ class ASH_EXPORT DockedWindowLayoutManager
   aura::Window* dock_container_;
   // Protect against recursive calls to Relayout().
   bool in_layout_;
+  // The former child being dragged.
+  aura::Window* dragged_former_child_;
   // The docked window being dragged.
   aura::Window* dragged_window_;
   // The launcher we are observing for launcher icon changes.
@@ -174,6 +179,10 @@ class ASH_EXPORT DockedWindowLayoutManager
   // Side of the screen that the dock is positioned at.
   DockedAlignment alignment_;
 
+  // The last active window. Used to maintain stacking even if no windows are
+  // currently focused.
+  aura::Window* last_active_;
+
   // Observers of dock bounds changes.
   ObserverList<DockedWindowLayoutManagerObserver> observer_list_;