Import w3c tests for the service workers

third_party/WebKit/LayoutTests/imported/wpt/service-workers/service-worker/resources/fetch-event-respond-with-stops-propagation-worker.js

 var result = null;
 
 self.addEventListener('message', function(event) {
     event.data.port.postMessage(result);
   });
 
 self.addEventListener('fetch', function(event) {
     if (!result)
       result = 'PASS';
     event.respondWith(new Response());
   });
 
 self.addEventListener('fetch', function(event) {
     result = 'FAIL: fetch event propagated';
   });