Don't assert when ServiceWorker::from is passed an empty promise

Source/modules/serviceworkers/ServiceWorker.cpp

 /*
  * 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 178 matching lines @@
 void ServiceWorker::onPromiseResolved()
 {
     if (m_proxyState == ContextStopped)
         return;
     setProxyState(Ready);
 }
 
 void ServiceWorker::waitOnPromise(ScriptPromise promise)
 {
     setProxyState(RegisterPromisePending);
-    promise.then(ThenFunction::create(this));
+    if (promise.isEmpty())
+        onPromiseResolved();
+    else
+        promise.then(ThenFunction::create(this));
 }
 
 bool ServiceWorker::hasPendingActivity() const
 {
     if (AbstractWorker::hasPendingActivity())
         return true;
     if (m_proxyState == ContextStopped)
         return false;
     return m_outerWorker->state() != blink::WebServiceWorkerStateDeactivated;
 }
@@ skipping 15 matching lines @@
     , WebServiceWorkerProxy(this)
     , m_outerWorker(worker)
     , m_proxyState(Initial)
 {
     ScriptWrappable::init(this);
     ASSERT(m_outerWorker);
     m_outerWorker->setProxy(this);
 }
 
 } // namespace WebCore