Reland of Clear LifecycleObserver::m_context when LifecycleObserver::contextDestroyed gets called

third_party/WebKit/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 "modules/serviceworkers/ServiceWorkerContainerClient.h"
 
 #include "core/dom/Document.h"
 #include "core/dom/ExecutionContext.h"
 #include "core/frame/LocalFrame.h"
 #include "core/loader/FrameLoaderClient.h"
@@ skipping 17 matching lines @@
 {
 }
 
 const char* ServiceWorkerContainerClient::supplementName()
 {
     return "ServiceWorkerContainerClient";
 }
 
 ServiceWorkerContainerClient* ServiceWorkerContainerClient::from(ExecutionContext* context)
 {
+    if (!context)
+        return nullptr;
     if (context->isWorkerGlobalScope()) {
         WorkerClients* clients = toWorkerGlobalScope(context)->clients();
         ASSERT(clients);
         return static_cast<ServiceWorkerContainerClient*>(Supplement<WorkerClients>::from(clients, supplementName()));
     }
     Document* document = toDocument(context);
     if (!document->frame())
         return nullptr;
 
     ServiceWorkerContainerClient* client = static_cast<ServiceWorkerContainerClient*>(Supplement<Document>::from(document, supplementName()));
     if (!client) {
         client = new ServiceWorkerContainerClient(document->frame()->loader().client()->createServiceWorkerProvider());
         Supplement<Document>::provideTo(*document, supplementName(), client);
     }
     return client;
 }
 
 void provideServiceWorkerContainerClientToWorker(WorkerClients* clients, std::unique_ptr<WebServiceWorkerProvider> provider)
 {
     clients->provideSupplement(ServiceWorkerContainerClient::supplementName(), ServiceWorkerContainerClient::create(std::move(provider)));
 }
 
 } // namespace blink