Test coverage for bug in WmWindowAura::OnWindowVisibilityChanged()

ash/common/wm_window_observer.h

Index: ash/common/wm_window_observer.h
diff --git a/ash/common/wm_window_observer.h b/ash/common/wm_window_observer.h
index a50b00e74e13336b8c2f8426dcc7e29d22c2a4c1..544fa3bc3c517f9f89a0c4562843a706597bdc44 100644
--- a/ash/common/wm_window_observer.h
+++ b/ash/common/wm_window_observer.h
@@ -46,6 +46,9 @@ class ASH_EXPORT WmWindowObserver {
                                      const gfx::Rect& new_bounds) {}
 
   virtual void OnWindowVisibilityChanging(WmWindow* window, bool visible) {}
+
+  // Behavior of this function matches that of
+  // aura::WindowObserver::OnWindowVisibilityChanged(), see it for details.
   virtual void OnWindowVisibilityChanged(WmWindow* window, bool visible) {}
 
   virtual void OnWindowTitleChanged(WmWindow* window) {}