[Service Workers] DCHECK from WebServiceWorkerResponse::visitHTTPHeaderFields.

third_party/WebKit/LayoutTests/http/tests/serviceworker/resources/iso-latin1-header-worker.js

 self.addEventListener('fetch', function(event) {
     var url = event.request.url;
     if (url.indexOf('dummy?test') == -1) {
       return;
     }
+
     event.respondWith(new Promise(function(resolve) {
         var headers = new Headers;
-        headers.append('foo', 'foo');
-        headers.append('foo', 'bar');
+        headers.append('TEST', 'ßÀ¿'); // header value holds the Latin1 (ISO8859-1) string.
         resolve(new Response('hello world', {'headers': headers}));
       }));
   });