Verify that the notification response contains sensible data

chrome/browser/notifications/notification_common.h

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CHROME_BROWSER_NOTIFICATIONS_NOTIFICATION_COMMON_H_
 #define CHROME_BROWSER_NOTIFICATIONS_NOTIFICATION_COMMON_H_
 
 namespace content {
 class BrowserContext;
 }  // namespace content
 
 // Shared functionality for both in page and persistent notification
 class NotificationCommon {
  public:
   // Things as user can do to a notification.
   enum Operation {
     CLICK = 0,
     CLOSE = 1,
-    SETTINGS = 2
+    SETTINGS = 2,
+    OPERATION_MAX = SETTINGS
   };
 
   // Open the Notification settings screen when clicking the right button.
   // TODO(miguelg) have it take a Profile instead once NotificationObjectProxy
   // is updated.
   static void OpenNotificationSettings(
       content::BrowserContext* browser_context);
 };
 
 #endif  // CHROME_BROWSER_NOTIFICATIONS_NOTIFICATION_COMMON_H_