Convert enable_pre_sync_backup flag to new flags system.

components/sync_driver/BUILD.gn

 # 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.
 
+import("//build/buildflag_header.gni")
 import("//build/config/features.gni")
 
 source_set("sync_driver") {
   sources = [
     "about_sync_util.cc",
     "about_sync_util.h",
     "backend_data_type_configurer.cc",
     "backend_data_type_configurer.h",
     "backend_migrator.cc",
     "backend_migrator.h",
@@ skipping 98 matching lines @@
     "sync_type_preference_provider.h",
     "sync_util.cc",
     "sync_util.h",
     "system_encryptor.cc",
     "system_encryptor.h",
     "ui_data_type_controller.cc",
     "ui_data_type_controller.h",
     "user_selectable_sync_type.h",
   ]
 
+  public_deps = [
+    ":features",
+  ]
   deps = [
     "//base",
     "//base:prefs",
     "//base/third_party/dynamic_annotations",
     "//components/data_use_measurement/core",
     "//components/invalidation/public",
     "//components/os_crypt",
     "//components/pref_registry",
     "//components/signin/core/browser",
     "//components/version_info",
@@ skipping 110 matching lines @@
   ]
 
   if (enable_configuration_policy) {
     sources += [ "sync_policy_handler_unittest.cc" ]
     deps += [
       "//components/policy",
       "//components/policy:policy_component",
     ]
   }
 }
+
+buildflag_header("features") {
+  header = "sync_driver_features.h"
+
+  # Whether to back up data before sync.
+  enable_pre_sync_backup = is_win || is_mac || (is_linux && !is_chromeos)
+
+  flags = [ "ENABLE_PRE_SYNC_BACKUP=$enable_pre_sync_backup" ]
+}