Fix some leaks and failures under valgrind in JS extensions unit tests.

extensions/test/data/unit_test_environment_specific_bindings.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 nativesPromise = requireAsync('testNatives');
-
 function registerHooks(api) {
-  var chromeTest = api.compiledApi;
   var apiFunctions = api.apiFunctions;
 
   apiFunctions.setHandleRequest('notifyPass', function() {
-    nativesPromise.then(function(natives) {
+    requireAsync('testNatives').then(function(natives) {
       natives.NotifyPass();
     });
   });
 
   apiFunctions.setHandleRequest('notifyFail', function(message) {
-    nativesPromise.then(function(natives) {
+    requireAsync('testNatives').then(function(natives) {
       natives.NotifyFail(message);
     });
   });
 
   apiFunctions.setHandleRequest('log', function() {
-    nativesPromise.then(function(natives) {
+    requireAsync('testNatives').then(function(natives) {
       natives.Log($Array.join(arguments, ' '));
     });
   });
 
 }
 
 function testDone(runNextTest) {
     // Use a promise here to allow previous test contexts to be eligible for
     // garbage collection.
     Promise.resolve().then(function() {
       runNextTest();
     });
 }
 
 function exportTests(tests, runTests, exports) {
   $Array.forEach(tests, function(test) {
     exports[test.name] = function() {
       runTests([test]);
       return true;
     }
   });
 }
 
 exports.registerHooks = registerHooks;
 exports.testDone = testDone;
 exports.exportTests = exportTests;