Use mojo from the PDF extension instead of using an extension API.

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/frame_interfaces',
-    'extensions/common/mojo/keep_alive.mojom',
-    'mojo/public/js/core',
-], function(frameInterfaces, mojom, core) {
-
-  /**
-   * An object that keeps the background page alive until closed.
-   * @constructor
-   * @alias module:keep_alive~KeepAlive
-   */
-  function KeepAlive() {
-    /**
-     * The handle to the keep-alive object in the browser.
-     * @type {!MojoHandle}
-     * @private
-     */
-    this.handle_ = frameInterfaces.getInterface(mojom.KeepAlive.name);
-  }
-
-  /**
-   * Removes this keep-alive.
-   */
-  KeepAlive.prototype.close = function() {
-    core.close(this.handle_);
-  };
-
-  var exports = {};
-
-  return {
-    /**
-     * Creates a keep-alive.
-     * @return {!module:keep_alive~KeepAlive} A new keep-alive.
-     */
-    createKeepAlive: function() { return new KeepAlive(); }
-  };
-});