[Mac] Fix a leak in CookiesWindowControllerTest by giving BrowserTestHelper a fake IO thread.

chrome/browser/cocoa/browser_test_helper.h

 // Copyright (c) 2009 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.
 
 #ifndef CHROME_BROWSER_COCOA_BROWSER_TEST_HELPER_H_
 #define CHROME_BROWSER_COCOA_BROWSER_TEST_HELPER_H_
 
 #include "chrome/browser/browser.h"
 #include "chrome/browser/chrome_thread.h"
 #include "chrome/browser/profile.h"
 #include "chrome/test/testing_profile.h"
 
 // Base class which contains a valid Browser*.  Lots of boilerplate to
 // recycle between unit test classes.
 //
 // TODO(jrg): move up a level (chrome/browser/cocoa -->
 // chrome/browser), and use in non-Mac unit tests such as
 // back_forward_menu_model_unittest.cc,
 // navigation_controller_unittest.cc, ..
 class BrowserTestHelper {
  public:
   BrowserTestHelper()
       : ui_thread_(ChromeThread::UI, &message_loop_),
-        file_thread_(ChromeThread::FILE, &message_loop_) {
+        file_thread_(ChromeThread::FILE, &message_loop_),
+        io_thread_(ChromeThread::IO, &message_loop_) {
     profile_.reset(new TestingProfile());
     profile_->CreateBookmarkModel(true);
     profile_->BlockUntilBookmarkModelLoaded();
 
     // TODO(shess): These are needed in case someone creates a browser
     // window off of browser_.  pkasting indicates that other
     // platforms use a stub |BrowserWindow| and thus don't need to do
     // this.
     // http://crbug.com/39725
     profile_->CreateAutocompleteClassifier();
@@ skipping 30 matching lines @@
     // |browser_| will be deleted by its BrowserWindowController.
     ignore_result(browser_.release());
   }
 
  private:
   scoped_ptr<TestingProfile> profile_;
   scoped_ptr<Browser> browser_;
   MessageLoopForUI message_loop_;
   ChromeThread ui_thread_;
   ChromeThread file_thread_;
+  ChromeThread io_thread_;
 };
 
 #endif  // CHROME_BROWSER_COCOA_BROWSER_TEST_HELPER_H_