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

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

 importScripts('worker-testharness.js');
 importScripts('../../resources/testharness-helpers.js');
 importScripts('test-helpers.js');
 
 var URL = 'https://www.example.com/test.html';
 
 function size(headers) {
   var count = 0;
   for (var header of headers) {
     ++count;
   }
   return count;
 }
 
 test(function() {
     var headers = new Headers;
     headers.set('User-Agent', 'Mozilla/5.0');
     headers.set('Accept', 'text/html');
     headers.set('X-ServiceWorker-Test', 'request test field');
 
     var request = new Request(URL, {method: 'GET', headers: headers});
 
     assert_equals(request.url, URL, 'Request.url should match');
     assert_equals(request.method, 'GET', 'Request.method should match');
-    assert_equals(request.referrer, location.href,
-                  'Request.referrer should match');
+    assert_equals(request.referrer, 'about:client',
+                  'Request.referrer should be about:client');
     assert_true(request.headers instanceof Headers,
                 'Request.headers should be Headers');
 
     // 'User-Agent' is a forbidden header.
     assert_equals(size(request.headers), 2,
                   'Request.headers size should match');
     // Note: detailed behavioral tests for Headers are in another test,
     // http/tests/serviceworker/headers.html.
 
     request.url = 'http://localhost/';
@@ skipping 449 matching lines @@
             result, expected_body,
             'Creating a Request with FormData body should success.');
         })
       .then(function() {
           t.done();
         })
       .catch(unreached_rejection(t));
     assert_true(request.bodyUsed,
                 'bodyUsed must be true after calling text()');
   }, 'Request body test in ServiceWorkerGlobalScope');