Create a stub NSUserNotificationCenter and make tests use it.

chrome/test/BUILD.gn

Index: chrome/test/BUILD.gn
diff --git a/chrome/test/BUILD.gn b/chrome/test/BUILD.gn
index 93e4cee6b25610b68690e0a87cd815c98815ca07..767e5ba0f9c5e4cbf1b7c78fdba182b80f63dd49 100644
--- a/chrome/test/BUILD.gn
+++ b/chrome/test/BUILD.gn
@@ -4143,6 +4143,8 @@ test("unit_tests") {
       "../browser/notifications/notification_permission_context_unittest.cc",
       "../browser/notifications/notification_platform_bridge_mac_unittest.mm",
       "../browser/notifications/platform_notification_service_unittest.cc",
+      "../browser/notifications/stub_notification_center_mac.h",
+      "../browser/notifications/stub_notification_center_mac.mm",
     ]
     if (is_android) {
       sources -= [