ServiceWorker: Spec alignment tweaks for Request/Response objects

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

 importScripts('worker-test-helpers.js');
 
 test(function() {
     var headers = new HeaderMap;
     headers.set('Content-Language', 'ja');
     headers.set('Content-Type', 'text/html; charset=UTF-8');
     headers.set('X-ServiceWorker-Test', 'response test field');
 
     var response = new Response(new Blob(), {
         status: 303,
         statusText: 'See Other',
         headers: headers
     });
 
     assert_equals(response.status, 303, 'Response.status should match');
     assert_equals(response.statusText, 'See Other', 'Response.statusText should match');
     assert_true(response.headers instanceof HeaderMap, 'Response.headers should be HeaderMap');
     assert_equals(response.headers.size, 3, 'Response.headers.size should match');
     // Note: detailed behavioral tests for HeaderMap are in another test,
     // http/tests/serviceworker/headermap.html.
 
+    response.status = 123;
+    response.statusText = 'Sesame Street';
+    assert_equals(response.status, 123, 'Response.status should be writable');
+    assert_equals(response.statusText, 'Sesame Street', 'Response.statusText should be writable');
+
+    assert_throws({name:'TypeError'}, function() { response.statusText = 'invalid \u0100'; },
+                  'Response.statusText should throw on invalid ByteString');
+
 }, 'Response in ServiceWorkerGlobalScope');