Fix and IPC fuzzer top crasher in OnWorkerScriptLoaded.

A compromised renderer can pass an arbitrary |provider_id|, so we should ensure that |provider_host| is not NULL and remove the DCHECK.

See https://cluster-fuzz.appspot.com/testcase?key=4713217552023552

R=falken@chromium.org,nhiroki@chromium.org

Review URL: https://codereview.chromium.org/901243002

Cr-Commit-Position: refs/heads/master@{#314881}

content/browser/service_worker/service_worker_dispatcher_host.cc

@@ -690,7 +690,11 @@ void ServiceWorkerDispatcherHost::OnWorkerScriptLoaded(
 
   ServiceWorkerProviderHost* provider_host =
       GetContext()->GetProviderHost(render_process_id_, provider_id);
-  DCHECK(provider_host);
+  if (!provider_host) {
+    BadMessageReceived();
+    return;
+  }
+
   provider_host->SetReadyToSendMessagesToWorker(thread_id);
 
   EmbeddedWorkerRegistry* registry = GetContext()->embedded_worker_registry();