service worker: Check for null GetResourceLoadTiming() for nav preloads.

This can return null per issue 817691. We only recently started getting crashes for
it being null for navigation preloads. It's possible this is due to
r636598 which allowed extensions to intercept nav preload requests.

Bug: 938639
Change-Id: I03149fa3c61234714611c272e4fa783948f2bf5c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1504594
Auto-Submit: Matt Falkenhagen <falken@chromium.org>
Reviewed-by: Nicolás Peña Moreno <npm@chromium.org>
Commit-Queue: Nicolás Peña Moreno <npm@chromium.org>
Cr-Commit-Position: refs/heads/master@{#637987}

third_party/blink/renderer/modules/service_worker/fetch_event.cc

@@ -192,10 +192,15 @@ void FetchEvent::OnNavigationPreloadComplete(
   resource_response.SetEncodedDataLength(encoded_data_length);
   resource_response.SetEncodedBodyLength(encoded_body_length);
   resource_response.SetDecodedBodyLength(decoded_body_length);
+
+  ResourceLoadTiming* timing = resource_response.GetResourceLoadTiming();
+  // |timing| can be null, see https://crbug.com/817691.
+  base::TimeTicks request_time =
+      timing ? timing->RequestTime() : base::TimeTicks();
   // According to the Resource Timing spec, the initiator type of
   // navigation preload request is "navigation".
-  scoped_refptr<ResourceTimingInfo> info = ResourceTimingInfo::Create(
-      "navigation", resource_response.GetResourceLoadTiming()->RequestTime());
+  scoped_refptr<ResourceTimingInfo> info =
+      ResourceTimingInfo::Create("navigation", request_time);
   info->SetNegativeAllowed(true);
   info->SetLoadFinishTime(completion_time);
   info->SetInitialURL(request_->url());