ServiceWorker: Implement navigator.serviceWorker.getRegistration [3/3]

Source/modules/serviceworkers/ServiceWorkerContainer.idl

 /*
  * Copyright (C) 2013 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 20 matching lines @@
 // https://slightlyoff.github.io/ServiceWorker/spec/service_worker/index.html#service-worker-container-interface
 [
     RuntimeEnabled=ServiceWorker,
     GarbageCollected
 ] interface ServiceWorkerContainer {
     [Unforgeable] readonly attribute ServiceWorker? controller;
     [CallWith=ScriptState] readonly attribute Promise ready;
 
     [CallWith=ScriptState, ImplementedAs=registerServiceWorker] Promise register(ScalarValueString url, optional Dictionary options);
     [CallWith=ScriptState, ImplementedAs=unregisterServiceWorker] Promise unregister(optional ScalarValueString scope = "/");
+    [CallWith=ScriptState] Promise getRegistration(optional ScalarValueString documentURL = "");
 };