Upstream service wrkr "respond with" tests to WPT

third_party/WebKit/LayoutTests/external/wpt/service-workers/service-worker/resources/fetch-event-respond-with-readable-stream-worker.js

+'use strict';
+
 self.addEventListener('fetch', event => {
     if (!event.request.url.match(/body-stream$/))
       return;
 
     const stream = new ReadableStream({start: controller => {
         const encoder = new TextEncoder();
         controller.enqueue(encoder.encode('PASS'));
         controller.close();
       }});
     event.respondWith(new Response(stream));
   });