Upstream foreign fetch tests.

third_party/WebKit/LayoutTests/external/wpt/service-workers/service-worker/resources/foreign-fetch-helper-script.js

 function handle_message(e) {
   fetch(e.data.url)
     .then(response => response.text())
     .then(text => e.ports[0].postMessage('Success: ' + text))
     .catch(error => e.ports[0].postMessage('Error: ' + error));
 }
 
 self.onmessage = handle_message;
 self.onconnect = e => {
   e.ports[0].onmessage = handle_message;
 };