Convert Service Worker layout tests to W3C testharness-style tests

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

 var port;
 
 // Exercise the 'onmessage' handler:
 self.onmessage = function(e) {
     var message = e.data;
     if ('port' in message) {
         port = message.port;
     }
 };
 
 // And an event listener:
 self.addEventListener('message', function(e) {
     var message = e.data;
     if ('value' in message) {
-        port.postMessage("Acking value: " + message.value);
+        port.postMessage('Acking value: ' + message.value);
     } else if ('done' in message) {
-        port.postMessage("quit");
+        port.postMessage('quit');
     }
 });