Add layout tests to check Accept request header values observed by Service Worker

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

 var requests = [];
 var port = undefined;
 
 self.onmessage = function(e) {
   var message = e.data;
   if ('port' in message) {
     port = message.port;
     port.postMessage({ready: true});
   }
 };
 
 self.addEventListener('fetch', function(event) {
     var url = event.request.url;
     if (url.indexOf('dummy?test') == -1) {
       return;
     }
     port.postMessage({
         url: url,
         mode: event.request.mode,
         redirect: event.request.redirect,
-        credentials: event.request.credentials
+        credentials: event.request.credentials,
+        accept: event.request.headers.get('accept')
       });
     event.respondWith(Promise.reject());
   });