Add null checks in navigator.serviceWorker access to fix possible crash

Source/modules/serviceworkers/ServiceWorkerContainerClient.cpp

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "config.h"
 #include "ServiceWorkerContainerClient.h"
 
 #include "core/dom/Document.h"
 #include "core/dom/ExecutionContext.h"
 #include "core/frame/LocalFrame.h"
@@ skipping 18 matching lines @@
 }
 
 ServiceWorkerContainerClient* ServiceWorkerContainerClient::from(ExecutionContext* context)
 {
     if (context->isDocument()) {
         Document* document = toDocument(context);
         ServiceWorkerContainerClient* client = static_cast<ServiceWorkerContainerClient*>(Supplement<Page>::from(document->page(), supplementName()));
         if (client)
             return client;
 
-        // If it's not provided yet create it lazily.
-        ASSERT(document->frame());
+        if (!document->frame())
+            return 0;
+
+        // If it's not provided yet, create it lazily.
         document->page()->provideSupplement(ServiceWorkerContainerClient::supplementName(), ServiceWorkerContainerClient::create(document->frame()->loader().client()->createServiceWorkerProvider()));
         return static_cast<ServiceWorkerContainerClient*>(Supplement<Page>::from(document->page(), supplementName()));
     }
 
     ASSERT(context->isWorkerGlobalScope());
     return static_cast<ServiceWorkerContainerClient*>(Supplement<WorkerClients>::from(toWorkerGlobalScope(context)->clients(), supplementName()));
 }
 
 ServiceWorkerContainerClient::ServiceWorkerContainerClient(PassOwnPtr<blink::WebServiceWorkerProvider> provider)
     : m_provider(provider)
 {
 }
 
 void provideServiceWorkerContainerClientToWorker(WorkerClients* clients, PassOwnPtr<blink::WebServiceWorkerProvider> provider)
 {
     clients->provideSupplement(ServiceWorkerContainerClient::supplementName(), ServiceWorkerContainerClient::create(provider));
 }
 
 } // namespace WebCore