Refactor event handler classes to make it easier to add new event type listeners.

chrome/browser/resources/chromeos/chromevox/common/chrome_extension_externs.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.
 
 
 /**
  * @fileoverview Definitions for the Chromium extensions API used by ChromeVox.
  *
  * @externs
  */
@@ skipping 267 matching lines @@
  * @typedef {{
  *   role: (!chrome.automation.RoleType|undefined),
  *   state: (Object|undefined),
  *   attributes: (Object|undefined)
  * }}
  */
 chrome.automation.FindParams;
 
 /**
  * @constructor
+ * @param {chrome.automation.EventType} type
+ * @param {chrome.automation.AutomationNode} node
  */
-chrome.automation.AutomationEvent = function() {};
+chrome.automation.AutomationEvent = function(type, node) {};
 
 /**
  * @type {!chrome.automation.AutomationNode}
  */
 chrome.automation.AutomationEvent.prototype.target;
 
 /**
  * @type {!chrome.automation.EventType}
  */
 chrome.automation.AutomationEvent.prototype.type;
@@ skipping 296 matching lines @@
 /**
  * @type {(Array<number>|undefined)}
  */
 chrome.automation.AutomationNode.prototype.lineBreaks;
 
 /**
  * @type {(number|undefined)}
  */
 chrome.automation.focusOffset;
 
+/**
+ * @type {(chrome.automation.AutomationNode|undefined)}
+ */
+chrome.automation.AutomationNode.prototype.activeDescendant;
+
 /** @type {function() : !Object} */
 chrome.app.getDetails;