Enable chrome.automation.getDesktop on all aura platforms.

chrome/test/data/extensions/api_test/automation/tests/desktop/desktop.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.
 
 var allTests = [
   function testGetDesktop() {
     chrome.automation.getDesktop(function(rootNode) {
       assertEq(RoleType.desktop, rootNode.role);
-      assertEq(RoleType.window, rootNode.firstChild.role);
+      assertEq(undefined, rootNode.firstChild);
       chrome.test.succeed();
     });
   },
 
   function testGetDesktopTwice() {
     var desktop = null;
     chrome.automation.getDesktop(function(rootNode) {
       desktop = rootNode;
     });
     chrome.automation.getDesktop(function(rootNode) {
       assertEq(rootNode, desktop);
       chrome.test.succeed();
     });
   },
 
   function testGetDesktopNested() {
     var desktop = null;
     chrome.automation.getDesktop(function(rootNode) {
       desktop = rootNode;
       chrome.automation.getDesktop(function(rootNode2) {
         assertEq(rootNode2, desktop);
         chrome.test.succeed();
       });
     });
   }
 ];
 
 chrome.test.runTests(allTests);