Enable chrome.automation.getDesktop on all aura platforms.

chrome/test/data/extensions/api_test/automation/tests/desktop/common.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 assertEq = chrome.test.assertEq;
 var assertFalse = chrome.test.assertFalse;
 var assertTrue = chrome.test.assertTrue;
 
 var EventType = chrome.automation.EventType;
 var RoleType = chrome.automation.RoleType;
@@ skipping 24 matching lines @@
     chrome.tabs.onUpdated.addListener(function(tabId, changeInfo) {
       if (tabId == tab.id && changeInfo.status == 'complete') {
         callback();
       }
     });
   });
 }
 
 function setupAndRunTests(allTests, opt_docString) {
   function runTestInternal() {
-    chrome.automation.getDesktop(function(rootNodeArg) {
-      rootNode = rootNodeArg;
-      chrome.test.runTests(allTests);
-    });
+    chrome.test.runTests(allTests);
   }
 
-  if (opt_docString)
-    runWithDocument(opt_docString, runTestInternal);
-  else
-    runTestInternal();
+  chrome.automation.getDesktop(function(rootNodeArg) {
+    rootNode = rootNodeArg;
+
+    // Only run the test when the window containing the new tab loads.
+    rootNodeArg.addEventListener(
+        chrome.automation.EventType.childrenChanged,
+        function(evt) {
+          var subroot = evt.target.firstChild;
+          if (!opt_docString || !subroot)
+            return;
+
+          if (subroot.role == 'rootWebArea' &&
+              subroot.attributes.docUrl.indexOf(opt_docString) != -1)
+            runTestInternal();
+        },
+        true);
+    if (opt_docString)
+      runWithDocument(opt_docString, null);
+    else
+      runTestInternal();
+  });
 }