Implement extension controlled indicators, starting with New Tab page

chrome/browser/resources/settings/on_startup_page/on_startup_browser_proxy.js

+// Copyright 2016 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.
+
+/** @typedef {{id: string, name: string, canBeDisabled: boolean}} */
+var NtpExtension;
+
+cr.define('settings', function() {
+  /** @interface */
+  function OnStartupBrowserProxy() {}
+
+  OnStartupBrowserProxy.prototype = {

[dpapad, 2016/11/17 01:16:46]

Do we need a new Proxy class, given that there is already
startup_urls_page_browser_proxy.js in the same folder? On a related note, I've
found the on_startup_page and startup_urls_page distinction unnecessary. Perhaps
we could simply merge those two elements to one, and then having on proxy for
both would be more natural.

[Dan Beam, 2016/11/17 01:33:11]

we probably could fold together, yes.  but right now this proxy and proxy and
associated handler have a 1:1:1 mapping (and specific purpose), which I don't
find bad.  it might also load less code in tests if we change the preload URL
(or maybe not if on_startup_page.html pulls in a lot of stuff)

+    /** @return {!Promise<?NtpExtension>} */
+    getNtpExtension: assertNotReached,
+  };
+
+  /**
+   * @constructor
+   * @implements {settings.OnStartupBrowserProxy}
+   */
+  function OnStartupBrowserProxyImpl() {}
+  cr.addSingletonGetter(OnStartupBrowserProxyImpl);
+
+  OnStartupBrowserProxyImpl.prototype = {
+    /** @override */
+    getNtpExtension: function() {
+      return cr.sendWithPromise('getNtpExtension');
+    },
+  };
+
+  return {
+    OnStartupBrowserProxy: OnStartupBrowserProxy,
+    OnStartupBrowserProxyImpl: OnStartupBrowserProxyImpl,
+  };
+});