Fix resource-timing.https.html test

This CL changes resource-timing-worker.js to use a generated response.
This means Chrome no longer passes the test.

Change-Id: Ib983e4fba9e81ddbee7f9572b6a06b3c1825577a
Reviewed-on: https://chromium-review.googlesource.com/1050383
Commit-Queue: Nicolás Peña Moreno <npm@chromium.org>
Reviewed-by: Timothy Dresser <tdresser@chromium.org>
Cr-Commit-Position: refs/heads/master@{#556964}

third_party/WebKit/LayoutTests/TestExpectations

@@ -4634,6 +4634,9 @@ crbug.com/800745 [ Linux ] virtual/gpu/fast/canvas/canvas-composite-video.html [
 
 crbug.com/832274 [ Linux ] fast/forms/search/search-appearance-basic.html [ Pass Failure ]
 
+crbug.com/840881 external/wpt/service-workers/service-worker/resource-timing.https.html [ Failure ]
+crbug.com/840881 virtual/outofblink-cors/external/wpt/service-workers/service-worker/resource-timing.https.html [ Failure ]
+
 # Sheriff 2018-04-12
 crbug.com/831993 [ Linux ] virtual/gpu-rasterization/images/cross-fade-overflow-position.html [ Pass Timeout ]
 

third_party/WebKit/LayoutTests/external/wpt/service-workers/service-worker/resources/resource-timing-worker.js

 self.addEventListener('fetch', function(event) {
-    if (event.request.url.indexOf('dummy.js') != -1) {
-      event.respondWith(fetch('empty.js'));
-    }
-  });
+  if (event.request.url.indexOf('dummy.js') != -1) {
+    event.respondWith(new Promise(resolve => {
+      // Slightly delay the response so we ensure we get a non-zero
+      // duration.
+      setTimeout(_ => resolve(new Response('// Empty javascript')), 50);
+    }));
+  }
+});