Create a stub NSUserNotificationCenter and make tests use it.

chrome/browser/notifications/stub_notification_center_mac.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_STUB_NOTIFICATION_CENTER_MAC_H_
+#define CHROME_BROWSER_NOTIFICATIONS_STUB_NOTIFICATION_CENTER_MAC_H_
+
+#import <AppKit/AppKit.h>
+
+@class NSUserNotificationCenterDelegate;
+
+// Stubs NSUserNotificationCenter so it can be used in tests without actually
+// displaying notifications.
+// Unlike the real class this is not a singleton and the lifecycle needs to be
+// handled by the caller.
+// See notification_platform_bridge_mac_unittest.mm for an example.
+@interface StubNotificationCenter : NSUserNotificationCenter
+- (instancetype)init;
+- (void)setDelegate:(NSUserNotificationCenterDelegate*)delegate;
+
+@end
+
+#endif  // CHROME_BROWSER_NOTIFICATIONS_STUB_NOTIFICATION_CENTER_MAC_H_