Upstream service worker postMessage tests to WPT

third_party/WebKit/LayoutTests/http/tests/serviceworker/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'});
-    }
-  });