Upstream service worker `postMessage` tests to WPT

third_party/WebKit/LayoutTests/http/tests/serviceworker/resources/postmessage-msgport-to-client-worker.js

-self.onmessage = function(e) {
-  e.waitUntil(self.clients.matchAll().then(function(clients) {
-      clients.forEach(function(client) {
-          var messageChannel = new MessageChannel();
-          messageChannel.port1.onmessage =
-            onMessageViaMessagePort.bind(null, messageChannel.port1);
-          client.postMessage(undefined, [messageChannel.port2]);
-        });
-    }));
-};
-
-function onMessageViaMessagePort(port, e) {
-  var message = e.data;
-  if ('value' in message) {
-    port.postMessage({ack: 'Acking value: ' + message.value});
-  } else if ('done' in message) {
-    port.postMessage({done: true});
-  }
-}