Abstract notification clicks in its own dictionary

chrome/browser/ui/cocoa/notifications/notification_constants_mac.mm

+// 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.
+
+#include "chrome/browser/ui/cocoa/notifications/notification_constants_mac.h"
+
+namespace notification_constants {
+
+// Applicable to NotificationBuilder and NotificationResponseBuilder
+NSString* const kNotificationOrigin = @"notificationOrigin";
+NSString* const kNotificationId = @"notificationId";
+NSString* const kNotificationProfileId = @"notificationProfileId";
+NSString* const kNotificationIncognito = @"notificationIncognito";
+
+// Only applicable to the NotificationResponseBuilder
+NSString* const kNotificationOperation = @"notificationOperation";
+NSString* const kNotificationButtonIndex = @"notificationButtonIndex";
+
+}  // notification_constants