Upstream service worker postMessage tests to WPT

third_party/WebKit/LayoutTests/external/wpt/service-workers/service-worker/ServiceWorkerGlobalScope/resources/postmessage-loopback-worker.js

 self.addEventListener('message', function(event) {
     if ('port' in event.data) {
       var port = event.data.port;
 
       var channel = new MessageChannel();
       channel.port1.onmessage = function(event) {
         if ('pong' in event.data)
           port.postMessage(event.data.pong);
       };
       self.registration.active.postMessage({ping: channel.port2},
                                            [channel.port2]);
     } else if ('ping' in event.data) {
       event.data.ping.postMessage({pong: 'OK'});
     }
   });