[Mac] Add a confirm to quit experiment to about:flags.

chrome/chrome_dll.gypi

 # Copyright (c) 2010 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.
 {
   'target_defaults': {
     'variables': {
       'chrome_dll_target': 0,
     },
     'target_conditions': [
       ['chrome_dll_target==1', {
@@ skipping 189 matching lines @@
                 'app/nibs/BookmarkBar.xib',
                 'app/nibs/BookmarkBarFolderWindow.xib',
                 'app/nibs/BookmarkBubble.xib',
                 'app/nibs/BookmarkEditor.xib',
                 'app/nibs/BookmarkNameFolder.xib',
                 'app/nibs/BrowserWindow.xib',
                 'app/nibs/ClearBrowsingData.xib',
                 'app/nibs/CollectedCookies.xib',
                 'app/nibs/Cookies.xib',
                 'app/nibs/CookieDetailsView.xib',
+                'app/nibs/ConfirmQuitPanel.xib',
                 'app/nibs/ContentBlockedCookies.xib',
                 'app/nibs/ContentBlockedImages.xib',
                 'app/nibs/ContentBlockedJavaScript.xib',
                 'app/nibs/ContentBlockedPlugins.xib',
                 'app/nibs/ContentBlockedPopups.xib',
                 'app/nibs/ContentBubbleGeolocation.xib',
                 'app/nibs/ContentExceptionsWindow.xib',
                 'app/nibs/ContentSettings.xib',
                 'app/nibs/DownloadItem.xib',
                 'app/nibs/DownloadShelf.xib',
@@ skipping 378 matching lines @@
           'configurations': {
             'Common_Base': {
               'msvs_target_platform': 'x64',
             },
           },
         },  # target chrome_dll
       ],
     }],
   ],
 }