Reimplement automation API on top of C++-backed AXTree.

chrome/test/data/extensions/api_test/automation/tests/tabs/tree_change.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.
 
 var allTests = [
   function testTreeChangedObserverForCreatingNode() {
     chrome.automation.addTreeChangeObserver(function(change) {
-      if (change.type == "nodeCreated" && change.target.name == "New") {
+      if (change.type == "subtreeCreated" && change.target.name == "New") {
         chrome.test.succeed();
       }
     });
 
     var addButton = rootNode.find({ attributes: { name: 'Add' }});
     addButton.doDefault();
   },
 
   function testTreeChangedObserverForRemovingNode() {
     chrome.automation.addTreeChangeObserver(function(change) {
       if (change.type == "nodeRemoved" && change.target.role == "listItem") {
         chrome.test.succeed();
       }
     });
 
     var removeButton = rootNode.find({ attributes: { name: 'Remove' }});
     removeButton.doDefault();
   }
 ];
 
 setUpAndRunTests(allTests, 'tree_change.html');