Import w3c tests for the service workers

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

 var result;
 
 self.addEventListener('message', function(event) {
     event.data.port.postMessage(result);
   });
 
 self.addEventListener('fetch', function(event) {
     setTimeout(function() {
         try {
           event.respondWith(new Response());
           result = 'FAIL: did not throw';
         } catch (error) {
           if (error.name == 'InvalidStateError')
             result = 'PASS';
           else
             result = 'FAIL: Unexpected exception: ' + error;
         }
       }, 0);
   });