Upstream service worker postMessage tests to WPT

third_party/WebKit/LayoutTests/external/wpt/service-workers/service-worker/ServiceWorkerGlobalScope/resources/postmessage-ping-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);
       };
 
       // Send a ping message to another service worker.
       self.registration.waiting.postMessage({ping: channel.port2},
                                             [channel.port2]);
     }
   });