Do not hide the web notification popups by the system tray, rather let them avoid (2nd)

ui/message_center/views/message_popup_collection.h

Index: ui/message_center/views/message_popup_collection.h
diff --git a/ui/message_center/views/message_popup_collection.h b/ui/message_center/views/message_popup_collection.h
index bba2dd65ce1ac04df60405fdd93078df7f40438a..8fa06f88c36ff00af63a13105c955ec53c668e72 100644
--- a/ui/message_center/views/message_popup_collection.h
+++ b/ui/message_center/views/message_popup_collection.h
@@ -29,6 +29,7 @@ class Widget;
 }
 
 namespace ash {
+class WebNotificationTrayTest;
 FORWARD_DECLARE_TEST(WebNotificationTrayTest, ManyPopupNotifications);
 }
 
@@ -109,6 +110,7 @@ class MESSAGE_CENTER_EXPORT MessagePopupCollection
                            ManyPopupNotifications);
   friend class test::MessagePopupCollectionTest;
   friend class test::MessagePopupCollectionWidgetsTest;
+  friend class ash::WebNotificationTrayTest;
   typedef std::list<ToastContentsView*> Toasts;
 
   void CloseAllWidgets();