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

LayoutTests/http/tests/serviceworker/request-end-to-end.html

Index: LayoutTests/http/tests/serviceworker/request-end-to-end.html
diff --git a/LayoutTests/http/tests/serviceworker/request-end-to-end.html b/LayoutTests/http/tests/serviceworker/request-end-to-end.html
index 0c05233edb208d9371290e4a8ceef98fd3fce67b..1d4dc61bc3cc2ee848e385cb80c5e6bcdc13f097 100644
--- a/LayoutTests/http/tests/serviceworker/request-end-to-end.html
+++ b/LayoutTests/http/tests/serviceworker/request-end-to-end.html
@@ -45,6 +45,8 @@ t.step(function() {
             'request.url should be passed to onfetch event.');
         assert_equals(event.data.method, 'GET',
                       'request.method should be passed to onfetch event.');
+        assert_equals(event.data.referrer, location.href,
+                      'request.referrer should be passed to onfetch event.');
         assert_equals(event.data.headers['user-agent'], navigator.userAgent,
                       'User-Agent header should be passed to onfetch event.')
         service_worker_unregister_and_done(t, scope);