Expose ServiceWorkerRegistration.showNotification() in Service Workers.

Source/modules/notifications/ServiceWorkerRegistrationNotifications.idl

Index: Source/modules/notifications/ServiceWorkerRegistrationNotifications.idl
diff --git a/Source/modules/notifications/ServiceWorkerRegistrationNotifications.idl b/Source/modules/notifications/ServiceWorkerRegistrationNotifications.idl
index c035165a14cbd559bbbc52218fc1d261b1d3e880..ae434b43c0de610ab6723542807ef03e2d579f28 100644
--- a/Source/modules/notifications/ServiceWorkerRegistrationNotifications.idl
+++ b/Source/modules/notifications/ServiceWorkerRegistrationNotifications.idl
@@ -5,6 +5,7 @@
 // https://notifications.spec.whatwg.org/#service-worker-api
 
 [
+    Exposed=(Window, ServiceWorker),
     RuntimeEnabled=ServiceWorkerNotifications
 ] partial interface ServiceWorkerRegistration {
     [CallWith=ScriptState] Promise showNotification(DOMString title, optional NotificationOptions options);