Implement ServiceWorker::postMessage() [Blink]

LayoutTests/http/tests/serviceworker/postmessage-worker.js

+var port;
+self.addEventListener('message', function(e) {
+
+    var message = e.data;
+
+    if ('port' in message) {
+        port = message.port;
+    } else if ('value' in message) {
+        port.postMessage("Acking value: " + message.value);
+    } else if ('done' in message) {
+        port.postMessage("quit");
+    }
+});