Convert enable_pre_sync_backup flag to new flags system.

components/browser_sync.gypi

 # Copyright 2015 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.
 
 {
   'targets': [
     {
       # GN version: //components/browser_sync_browser
       'target_name': 'browser_sync_browser',
       'type': 'static_library',
@@ skipping 12 matching lines @@
         'history_core_browser',
         'invalidation_impl',
         'invalidation_public',
         'keyed_service_core',
         'password_manager_sync_browser',
         'pref_registry',
         'signin_core_browser',
         'syncable_prefs',
         'sync_bookmarks',
         'sync_driver',
+        'sync_driver_features',
         'sync_sessions',
         'variations',
         'version_info',
       ],
       'include_dirs': [
         '..',
       ],
       'sources': [
         # Note: file list duplicated in GN build.
         'browser_sync/browser/profile_sync_components_factory_impl.cc',
@@ skipping 15 matching lines @@
         '..',
       ],
       'sources': [
         # Note: file list duplicated in GN build.
         'browser_sync/common/browser_sync_switches.cc',
         'browser_sync/common/browser_sync_switches.h',
       ],
     },
   ],
 }