[ServiceWorker] Serialize request.referrer to "about:client" when it's client.

LayoutTests/http/tests/serviceworker/resources/request-end-to-end-worker.js

 var port = undefined;
 
 onmessage = function(e) {
   var message = e.data;
   if (typeof message === 'object' && 'port' in message) {
     port = message.port;
   }
 };
 
 onfetch = function(e) {
   var headers = {};
   for (var header of e.request.headers) {
     var key = header[0], value = header[1];
     headers[key] = value;
   }
   port.postMessage({
       url: e.request.url,
       method: e.request.method,
+      referrer: e.request.referrer,
       headers: headers,
       headerSize: e.request.headers.size
     });
 };