Make ServiceWorker::from handle "unwrapping" a null ServiceWorker

Currently ServiceWorkers are non-null, however when we implement the .waiting property, the browser will "set" a null ServiceWorker to clear the property. Make Blink ready to unwrap one of those.

BUG=379012

Review URL: https://codereview.chromium.org/323523002

git-svn-id: svn://svn.chromium.org/blink/trunk@175853 bbb929c8-8fbe-4397-9dbb-9b2b20218538

third_party/WebKit/Source/modules/serviceworkers/ServiceWorker.cpp

@@ -140,6 +140,9 @@ const AtomicString& ServiceWorker::state() const
 
 PassRefPtr<ServiceWorker> ServiceWorker::from(ExecutionContext* executionContext, WebType* worker)
 {
+    if (!worker)
+        return PassRefPtr<ServiceWorker>();
+
     blink::WebServiceWorkerProxy* proxy = worker->proxy();
     ServiceWorker* existingServiceWorker = proxy ? proxy->unwrap() : 0;
     if (existingServiceWorker) {