DOMWindow::navigator should return navigator w/o frame when detached.

Source/modules/serviceworkers/NavigatorServiceWorker.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 "modules/serviceworkers/NavigatorServiceWorker.h"
 
 #include "core/dom/Document.h"
 #include "core/frame/LocalDOMWindow.h"
 #include "core/frame/LocalFrame.h"
 #include "core/frame/Navigator.h"
 #include "modules/serviceworkers/ServiceWorkerContainer.h"
 
 namespace blink {
 
 NavigatorServiceWorker::NavigatorServiceWorker(Navigator& navigator)
     : DOMWindowProperty(navigator.frame())
 {
 }
 
 NavigatorServiceWorker::~NavigatorServiceWorker()
 {
 }
 
 NavigatorServiceWorker* NavigatorServiceWorker::from(Document& document)
 {
     if (!document.frame() || !document.frame()->domWindow())
         return nullptr;
-    Navigator& navigator = *document.frame()->domWindow()->navigator();
-    return &from(navigator);
+    Navigator* navigator = document.frame()->domWindow()->navigator();
+    if (!navigator)
+        return nullptr;
+    return &from(*navigator);
 }
 
 NavigatorServiceWorker& NavigatorServiceWorker::from(Navigator& navigator)
 {
     NavigatorServiceWorker* supplement = toNavigatorServiceWorker(navigator);
     if (!supplement) {
         supplement = new NavigatorServiceWorker(navigator);
         provideTo(navigator, supplementName(), supplement);
         if (navigator.frame() && navigator.frame()->securityContext()->securityOrigin()->canAccessServiceWorkers()) {
             // Initialize ServiceWorkerContainer too.
@@ skipping 43 matching lines @@
 }
 
 DEFINE_TRACE(NavigatorServiceWorker)
 {
     visitor->trace(m_serviceWorker);
     HeapSupplement<Navigator>::trace(visitor);
     DOMWindowProperty::trace(visitor);
 }
 
 } // namespace blink