Create a stub NSUserNotificationCenter and make tests use it.

chrome/browser/notifications/notification_platform_bridge_mac.mm

 // Copyright 2015 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/notifications/notification_platform_bridge_mac.h"
 
 #include <utility>
 
 #include "base/bind.h"
 #include "base/bind_helpers.h"
@@ skipping 265 matching lines @@
     const std::string& profile_id,
     bool incognito,
     std::set<std::string>* notifications) const {
   DCHECK(notifications);
 
   NSString* current_profile_id = base::SysUTF8ToNSString(profile_id);
   for (NSUserNotification* toast in
        [notification_center_ deliveredNotifications]) {
     NSString* toast_profile_id = [toast.userInfo
         objectForKey:notification_constants::kNotificationProfileId];
-    if (toast_profile_id == current_profile_id) {
+    if ([toast_profile_id isEqualToString:current_profile_id]) {
       notifications->insert(base::SysNSStringToUTF8([toast.userInfo
           objectForKey:notification_constants::kNotificationId]));
     }
   }
   return true;
 }
 
 // static
 void NotificationPlatformBridgeMac::ProcessNotificationResponse(
     NSDictionary* response) {
@@ skipping 178 matching lines @@
   [[xpcConnection_ remoteObjectProxy] closeAllNotifications];
 }
 
 // NotificationReply implementation
 - (void)notificationClick:(NSDictionary*)notificationResponseData {
   NotificationPlatformBridgeMac::ProcessNotificationResponse(
       notificationResponseData);
 }
 
 @end