Enable chrome.automation.getDesktop on all aura platforms.

chrome/test/data/extensions/api_test/automation/tests/desktop/lazy_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(arg) {
-      assertEq(undefined, arg);
-      assertEq('getDesktop is unsupported by this platform',
-               chrome.runtime.lastError.message);
-      chrome.test.succeed();
+    chrome.automation.getDesktop(function(r) {
+      chrome.windows.create({}, function() {
+        assertEq(1, r.children.length);
+        chrome.test.succeed();
+      });
     });
   }
 ];
 
 chrome.test.runTests(allTests);