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

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

+var values = [];
+
+self.onmessage = function(e) {
+    self.clients.getServiced().then(function(clients) {
+        clients.forEach(function(client) {
+            client.postMessage('Sending message via clients');
+            client.postMessage('quit');
+        });
+    });
+};