ServiceWorker: Add layout test for WindowClient.navigate().

third_party/WebKit/LayoutTests/http/tests/serviceworker/resources/windowclient-navigate-worker.js

+var port = null;
+
+self.onmessage = function(e) {
+  port = e.data.port;
+  port.onmessage = on_message;
+  port.postMessage('ready');
+};
+
+function on_message(e) {
+  var url = e.data;
+  var client;
+
+  self.clients.matchAll({ includeUncontrolled : true }).then(function(cs) {
+        cs.forEach(function(c) { c.frameType == 'nested' && (client = c); });
+        return client.navigate(url);
+      })
+    .then(function(c) {
+        if (!c)
+          port.postMessage(c);
+        else
+          port.postMessage(c.url);
+      })
+    .catch(function(e) {
+        port.postMessage(e.name);
+      });
+}