[ServiceWorker] Set FetchEvent's request's headers' guard to immutable

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 1d4dc61bc3cc2ee848e385cb80c5e6bcdc13f097..8e459cf1d14f4520620bd0b656977727a108925d 100644
--- a/LayoutTests/http/tests/serviceworker/request-end-to-end.html
+++ b/LayoutTests/http/tests/serviceworker/request-end-to-end.html
@@ -41,7 +41,8 @@ t.step(function() {
     function onMessage(event) {
         assert_equals(
             event.data.url,
-            location.href.substring(0, location.href.lastIndexOf('/') + 1) + scope,
+            location.href.substring(0, location.href.lastIndexOf('/') + 1) +
+            scope,
             'request.url should be passed to onfetch event.');
         assert_equals(event.data.method, 'GET',
                       'request.method should be passed to onfetch event.');
@@ -49,6 +50,9 @@ t.step(function() {
                       '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.')
+        assert_equals(event.data.errorNameWhileAppendingHeader, 'TypeError',
+                      'Appending a new header to the request must throw a ' +
+                      'TypeError.')
         service_worker_unregister_and_done(t, scope);
     }
 });