Convert Service Worker layout tests to W3C testharness-style tests

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

-var port;
-
-// Exercise the 'onmessage' handler:
-self.onmessage = function(e) {
-    var message = e.data;
-    if ('port' in message) {
-        port = message.port;
-    }
-};
-
-// And an event listener:
-self.addEventListener('message', function(e) {
-    var message = e.data;
-    if ('value' in message) {
-        port.postMessage("Acking value: " + message.value);
-    } else if ('done' in message) {
-        port.postMessage("quit");
-    }
-});