Add an extensions keep-alive client.

extensions/renderer/resources/keep_alive.js

+// Copyright 2014 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+define('keep_alive', [
+    'content/public/renderer/service_provider',
+    'extensions/common/mojo/keep_alive.mojom',
+    'mojo/public/js/bindings/router',
+], function(serviceProvider, mojom, routerModule) {
+
+  function KeepAlive() {

[Ken Rockot(use gerrit already), 2014/10/24 18:29:55]

Documentation? It would be nice (IMO) to have our JS code littered with
appropriate type annotations, but I actually don't know how to name types in
this require-js universe.

What is the qualified name of the mojo Router type? Is it
mojo.public.js.bindings.router.Router? Should it be assigned via annotation in
router.js?

Anyway not looking to answer these questions here, but we should be thinking
about it. Compileable JS throughout Chrome is a very worthy goal to keep in
mind.

[Sam McNally, 2014/10/27 05:07:24]

Done. (Switched to holding the handle directly since we never send or receive
any messages with it).

I'm not sure how naming works, but I would guess
module:mojo/public/js/bindings/router~Router. I expect router.js would need to
be annotated for it to be useful.

+    this.router_ = new routerModule.Router(
+        serviceProvider.connectToService(mojom.KeepAlive.name));
+  }
+
+  KeepAlive.prototype.close = function() {
+    this.router_.close();
+  };
+
+  return {
+    createKeepAlive: function() { return new KeepAlive(); },
+  };
+});