MD Settings: Convert all browser proxies to use ES6 class syntax.

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