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

chrome/browser/cocoa/browser_test_helper.h

Index: chrome/browser/cocoa/browser_test_helper.h
diff --git a/chrome/browser/cocoa/browser_test_helper.h b/chrome/browser/cocoa/browser_test_helper.h
index 77016f1b9a3d88c4d479827b26bc73d81298ed0b..ab1f675c5bfe235f609f87d66e8043fe9a93781b 100644
--- a/chrome/browser/cocoa/browser_test_helper.h
+++ b/chrome/browser/cocoa/browser_test_helper.h
@@ -21,7 +21,8 @@ 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();
@@ -72,6 +73,7 @@ class BrowserTestHelper {
   MessageLoopForUI message_loop_;
   ChromeThread ui_thread_;
   ChromeThread file_thread_;
+  ChromeThread io_thread_;
 };
 
 #endif  // CHROME_BROWSER_COCOA_BROWSER_TEST_HELPER_H_