Add some tests to make sure message ports properly queue up message.

LayoutTests/http/tests/serviceworker/chromium/resources/postmessage-port-worker.js

+var port;
+
+self.onmessage = function(e) {
+  var message = e.data;
+  if ('port' in message) {
+    message.port.onmessage = function(e2) {
+      port = e2.data.port;
+      port.onmessage = on_message;
+    };
+  }
+};
+
+function on_message(e) {
+  var message = e.data;
+  if ('value' in message) {
+    port.postMessage('Acking value: ' + message.value);
+  } else if ('done' in message) {
+    port.postMessage('quit');
+  }
+}