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

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;
   }
+  var errorNameWhileAddingHeader = '';
+  try {
+    e.request.headers.append('Test-Header', 'TestValue');
+  } catch (e) {
+    errorNameWhileAppendingHeader = e.name;
+  }
   port.postMessage({
       url: e.request.url,
       method: e.request.method,
       referrer: e.request.referrer,
       headers: headers,
-      headerSize: e.request.headers.size
+      headerSize: e.request.headers.size,
+      errorNameWhileAppendingHeader: errorNameWhileAppendingHeader
     });
 };