Don't set lastError if chrome.dial.discoverNow() can't start.

chrome/test/data/extensions/api_test/dial/experimental/discovery.js

-// Copyright (c) 2012 The Chromium Authors. All rights reserved.
+// Copyright (c) 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.
 
 onload = function() {
   chrome.test.runTests([
-    function discovery() {
-      var discoverNowShouldFail = function(result) {
-        if (!result) {
-          chrome.test.succeed();
-        } else {
-          chrome.test.fail();
-        }
+    function discoverySucceeds() {
+      var discoverNowCallback = function(result) {
+        chrome.test.assertNoLastError();
+        // The result may be true or false depending on whether periodic
+        // discovery is running at the same time.
+        // TODO(mfoltz): We may want to update the API to distinguish error cases
+        // from simultaneous discovery.
+        chrome.test.assertTrue(typeof(result) == 'boolean');
+        chrome.test.succeed();
       };
-      chrome.dial.discoverNow(discoverNowShouldFail);
+      chrome.dial.onDeviceList.addListener(function(deviceList) {});
+      chrome.dial.discoverNow(discoverNowCallback);
     }
   ]);
 };