Add blink-side binding code and tests for ServiceWorker -> Document postMessage (3/3)

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

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