Convert enable_pre_sync_backup flag to new flags system.

components/sync_driver.gypi

 # 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.
 
 {
   'targets': [
     {
       # GN version: //components/sync_driver
       'target_name': 'sync_driver',
       'type': 'static_library',
       'dependencies': [
         '../base/base.gyp:base',
         '../net/net.gyp:net',
         '../sync/sync.gyp:sync',
         '../third_party/cacheinvalidation/cacheinvalidation.gyp:cacheinvalidation',
         '../third_party/cacheinvalidation/cacheinvalidation.gyp:cacheinvalidation_proto_cpp',
         'data_use_measurement_core',
         'invalidation_public',
         'os_crypt',
         'signin_core_browser',
+        'sync_driver_features',
         'version_info',
       ],
       'include_dirs': [
         '..',
       ],
       'sources': [
         # Note: file list duplicated in GN build.
         'sync_driver/about_sync_util.cc',
         'sync_driver/about_sync_util.h',
         'sync_driver/backend_data_type_configurer.cc',
@@ skipping 114 matching lines @@
             'policy_component',
           ],
           'sources': [
             'sync_driver/sync_policy_handler.cc',
             'sync_driver/sync_policy_handler.h',
           ],
         }],
       ],
     },
     {
+      # GN version: //components/sync_driver:features
+      'target_name': 'sync_driver_features',
+      'includes': [ '../build/buildflag_header.gypi' ],
+      'variables': {
+        'buildflag_header_path': 'components/sync_driver/sync_driver_features.h',
+        'variables': {
+          'conditions': [
+            ['OS=="win" or OS=="mac" or (OS=="linux" and chromeos==0)', {
+              'enable_pre_sync_backup%': 1,
+            }, {
+              'enable_pre_sync_backup%': 0,
+            }],
+          ],
+        },
+        'buildflag_flags': [
+          "ENABLE_PRE_SYNC_BACKUP=<(enable_pre_sync_backup)",
+        ],
+      },
+    },
+    {
       'target_name': 'sync_driver_test_support',
       'type': 'static_library',
       'dependencies': [
         'sync_driver',
         'version_info',
         '../base/base.gyp:base',
         '../sync/sync.gyp:sync',
         '../sync/sync.gyp:test_support_sync_internal_api',
         '../testing/gmock.gyp:gmock',
         '../testing/gtest.gyp:gtest',
@@ skipping 29 matching lines @@
         'sync_driver/non_frontend_data_type_controller_mock.cc',
         'sync_driver/non_frontend_data_type_controller_mock.h',
         'sync_driver/non_ui_data_type_controller_mock.cc',
         'sync_driver/non_ui_data_type_controller_mock.h',
         'sync_driver/sync_api_component_factory_mock.cc',
         'sync_driver/sync_api_component_factory_mock.h',
       ],
     },
   ],
 }