Service Worker: Add ServiceWorkerContainer.getRegistrations() method.

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 21 matching lines @@
 [
     RuntimeEnabled=ServiceWorker,
     GarbageCollected,
     TypeChecking=Interface,
 ] interface ServiceWorkerContainer : EventTarget {
     [Unforgeable] readonly attribute ServiceWorker? controller;
     [CallWith=ScriptState] readonly attribute Promise<ServiceWorkerRegistration> ready;
 
     [CallWith=ScriptState, ImplementedAs=registerServiceWorker] Promise<ServiceWorkerRegistration> register(USVString url, optional RegistrationOptions options);
     [CallWith=ScriptState] Promise<ServiceWorkerRegistration> getRegistration(optional USVString documentURL = "");
+    [CallWith=ScriptState] Promise<sequence<ServiceWorkerRegistration>> getRegistrations();
 
     attribute EventHandler oncontrollerchange;
 };