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

chrome/browser/resources/chromeos/chromevox/cvox2/background/tabs_automation_handler.js

 // Copyright 2015 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 Handles automation from a tabs automation node.
  */
 
 goog.provide('TabsAutomationHandler');
 
 goog.require('DesktopAutomationHandler');
 
 goog.scope(function() {
 var EventType = chrome.automation.EventType;
 var RoleType = chrome.automation.RoleType;
 
 /**
  * @param {!chrome.automation.AutomationNode} tabRoot
  * @constructor
  * @extends {DesktopAutomationHandler}
  */
 TabsAutomationHandler = function(tabRoot) {
   DesktopAutomationHandler.call(this, tabRoot);
 
   if (tabRoot.role != RoleType.rootWebArea)
     throw new Error('Expected rootWebArea node but got ' + tabRoot.role);
 
   // When the root is focused, simulate what happens on a load complete.
-  if (tabRoot.state.focused)
-    this.onLoadComplete({target: tabRoot, type: EventType.loadComplete});
+  if (tabRoot.state.focused) {
+    this.onLoadComplete(
+        new chrome.automation.AutomationEvent(EventType.loadComplete, tabRoot));
+  }
 };
 
 TabsAutomationHandler.prototype = {
   __proto__: DesktopAutomationHandler.prototype,
 
   /** @override */
   didHandleEvent_: function(evt) {
     evt.stopPropagation();
   },
 
   /** @override */
   onLoadComplete: function(evt) {
     ChromeVoxState.instance.refreshMode(evt.target.docUrl);
     var focused = evt.target.find({state: {focused: true}}) || evt.target;
-    this.onFocus({target: focused, type: EventType.focus});
+    this.onFocus(new chrome.automation.AutomationEvent(
+        EventType.focus, focused));
   }
 };
 
 });  // goog.scope