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

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

Index: chrome/test/data/extensions/api_test/dial/experimental/discovery_no_listeners.js
diff --git a/chrome/test/data/extensions/api_test/dial/experimental/discovery.js b/chrome/test/data/extensions/api_test/dial/experimental/discovery_no_listeners.js
similarity index 58%
copy from chrome/test/data/extensions/api_test/dial/experimental/discovery.js
copy to chrome/test/data/extensions/api_test/dial/experimental/discovery_no_listeners.js
index 917462c41ce488ca7ee30b460dc56aa49047dba2..b0ad2aeb619fa1d4e176fdd5038cf5f1a11ef11d 100644
--- a/chrome/test/data/extensions/api_test/dial/experimental/discovery.js
+++ b/chrome/test/data/extensions/api_test/dial/experimental/discovery_no_listeners.js
@@ -4,15 +4,17 @@
 
 onload = function() {
   chrome.test.runTests([
-    function discovery() {
-      var discoverNowShouldFail = function(result) {
+    function discoverNowWithoutListeners() {
+      var discoverNowCallback = function(result) {
+        chrome.test.assertNoLastError();
+        chrome.test.assertTrue(typeof(result) == 'boolean');
         if (!result) {
           chrome.test.succeed();
         } else {
           chrome.test.fail();
         }
       };
-      chrome.dial.discoverNow(discoverNowShouldFail);
+      chrome.dial.discoverNow(discoverNowCallback);
     }
   ]);
 };