Create a stub NSUserNotificationCenter and make tests use it.

chrome/browser/notifications/notification_platform_bridge_mac.mm

Index: chrome/browser/notifications/notification_platform_bridge_mac.mm
diff --git a/chrome/browser/notifications/notification_platform_bridge_mac.mm b/chrome/browser/notifications/notification_platform_bridge_mac.mm
index b32e39cd182ad5dbec3267aa2521822e1b80c338..e8f9f1cbc6e15ca045b0a9ba15fd7c33a7436917 100644
--- a/chrome/browser/notifications/notification_platform_bridge_mac.mm
+++ b/chrome/browser/notifications/notification_platform_bridge_mac.mm
@@ -283,7 +283,7 @@ bool NotificationPlatformBridgeMac::GetDisplayed(
        [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]));
     }