Fixes bug in WmWindowAura::OnWindowVisibilityChanged()

ui/aura/window_observer.h

Index: ui/aura/window_observer.h
diff --git a/ui/aura/window_observer.h b/ui/aura/window_observer.h
index f3123c05db87515901605cd6def98b8faf40ef4e..9a271cf38630acbc0350d82d8195a4bd1c6a25fc 100644
--- a/ui/aura/window_observer.h
+++ b/ui/aura/window_observer.h
@@ -70,6 +70,12 @@ class AURA_EXPORT WindowObserver {
   // value supplied to SetVisible(). If |visible| is true, window->IsVisible()
   // may still return false. See description in Window::IsVisible() for details.
   virtual void OnWindowVisibilityChanging(Window* window, bool visible) {}
+
+  // When the visibility of a Window changes OnWindowVisibilityChanged() is
+  // called for all observers attached to descendants of the Window as well
+  // as all observers attached to ancestors of the Window. The Window supplied
+  // to OnWindowVisibilityChanged() is the Window that Show()/Hide() was called
+  // on.
   virtual void OnWindowVisibilityChanged(Window* window, bool visible) {}
 
   // Invoked when SetBounds() is invoked on |window|. |old_bounds| and