Add new flag for Notification Center features that are not ready yet.

ui/message_center/message_center_switches.cc

Index: ui/message_center/message_center_switches.cc
diff --git a/ui/message_center/message_center_switches.cc b/ui/message_center/message_center_switches.cc
index 99781ec97ab95cb2090eb4d89ccd8d0c00d2c19f..8d374f7e29a1644e1251a53ece0151ec5f7e5533 100644
--- a/ui/message_center/message_center_switches.cc
+++ b/ui/message_center/message_center_switches.cc
@@ -10,5 +10,8 @@ namespace switches {
 const char kDisableRichNotifications[] = "disable-rich-notifications";
 const char kEnableRichNotifications[] = "enable-rich-notifications";
 
+const char kEnableExperimentalNotificationUI[] =
+    "enable-experimental-notification-ui";
+
 }  // namespace switches
 }  // namespace message_center