Fix race condition in ExtensionApiTest.Messaging.

chrome.tabs.onUpdated may fire when the listener is not set up yet.

BUG=

Review URL: https://codereview.chromium.org/537953002

Cr-Commit-Position: refs/heads/master@{#293637}

chrome/test/data/extensions/api_test/messaging/connect/test.js

@@ -28,18 +28,20 @@ chrome.test.getConfig(function(config) {
   chrome.test.runTests([
     function setupTestTab() {
       chrome.test.log("Creating tab...");
-      chrome.tabs.create({
-        url: "http://localhost:PORT/extensions/test_file.html"
-                 .replace(/PORT/, config.testServer.port)
-      }, function(newTab) {
+      var newTab = null;
       var doneListening = listenForever(chrome.tabs.onUpdated,
         function(_, info, tab) {
-            if (tab.id == newTab.id && info.status == 'complete') {
+          if (newTab && tab.id == newTab.id && info.status == 'complete') {
             chrome.test.log("Created tab: " + tab.url);
             testTab = tab;
             doneListening();
           }
         });
+      chrome.tabs.create({
+        url: "http://localhost:PORT/extensions/test_file.html"
+                 .replace(/PORT/, config.testServer.port)
+      }, function(tab) {
+        newTab = tab;
       });
     },