Add layout tests to check that a FetchEvent includes the right headers

third_party/WebKit/LayoutTests/http/tests/serviceworker/resources/fetch-event-test-worker.js

 function handleString(event) {
   event.respondWith(new Response('Test string'));
 }
 
 function handleBlob(event) {
   event.respondWith(new Response(new Blob(['Test blob'])));
 }
 
 function handleReferrer(event) {
   event.respondWith(new Response(new Blob(
@@ skipping 52 matching lines @@
     event.respondWith(fetch('other.html').then(function(response) {
         lastResponseForUsedCheck = response;
         return response;
       }));
   } else {
     event.respondWith(new Response(
         'bodyUsed: ' + lastResponseForUsedCheck.bodyUsed));
   }
 }
 
+function handleHeaders(event) {
+  const headers = Array.from(event.request.headers);
+  event.respondWith(new Response(JSON.stringify(headers)));
+}
+
 self.addEventListener('fetch', function(event) {
     var url = event.request.url;
     var handlers = [
       { pattern: '?string', fn: handleString },
       { pattern: '?blob', fn: handleBlob },
       { pattern: '?referrer', fn: handleReferrer },
       { pattern: '?clientId', fn: handleClientId },
       { pattern: '?ignore', fn: function() {} },
       { pattern: '?null', fn: handleNullBody },
       { pattern: '?fetch', fn: handleFetch },
       { pattern: '?form-post', fn: handleFormPost },
       { pattern: '?multiple-respond-with', fn: handleMultipleRespondWith },
-      { pattern: '?used-check', fn: handleUsedCheck }
+      { pattern: '?used-check', fn: handleUsedCheck },
+      { pattern: '?headers', fn: handleHeaders }
     ];
 
     var handler = null;
     for (var i = 0; i < handlers.length; ++i) {
       if (url.indexOf(handlers[i].pattern) != -1) {
         handler = handlers[i];
         break;
       }
     }
 
     if (handler) {
       handler.fn(event);
     } else {
       event.respondWith(new Response(new Blob(
         ['Service Worker got an unexpected request: ' + url])));
     }
   });