Removes unused parameter from RootWindow::OnWindowHidden.

ui/aura/root_window.h

Index: ui/aura/root_window.h
diff --git a/ui/aura/root_window.h b/ui/aura/root_window.h
index 0e40d8a2f5f5b5c86bedd5de39d660410217dd02..e8a6b260f85ae1a241702343b6daa349d102ece7 100644
--- a/ui/aura/root_window.h
+++ b/ui/aura/root_window.h
@@ -338,11 +338,8 @@ class AURA_EXPORT RootWindow : public ui::CompositorDelegate,
 
   // Called when a window becomes invisible, either by being removed
   // from root window hierarchy, via SetVisible(false) or being destroyed.
-  // |reason| specifies what triggered the hiding. |new_root| is the new root
-  // window, and may be NULL.
-  void OnWindowHidden(Window* invisible,
-                      WindowHiddenReason reason,
-                      RootWindow* new_root);
+  // |reason| specifies what triggered the hiding.
+  void OnWindowHidden(Window* invisible, WindowHiddenReason reason);
 
   // Cleans up the gesture recognizer for all windows in |window| (including
   // |window| itself).