Convert enable_pre_sync_backup flag to new flags system.

build/config/BUILD.gn

 # Copyright (c) 2013 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/config/allocator.gni")
 import("//build/config/chrome_build.gni")
 import("//build/config/crypto.gni")
 import("//build/config/features.gni")
 import("//build/config/sanitizers/sanitizers.gni")
 import("//build/config/ui.gni")
@@ skipping 221 matching lines @@
   }
   if (proprietary_codecs) {
     defines += [ "USE_PROPRIETARY_CODECS" ]
   }
   if (enable_hangout_services_extension) {
     defines += [ "ENABLE_HANGOUT_SERVICES_EXTENSION=1" ]
   }
   if (enable_background) {
     defines += [ "ENABLE_BACKGROUND=1" ]
   }
-  if (enable_pre_sync_backup) {
-    defines += [ "ENABLE_PRE_SYNC_BACKUP" ]
-  }
   if (enable_video_hole) {
     defines += [ "VIDEO_HOLE=1" ]
   }
   if (safe_browsing_mode == 1) {
     defines += [ "FULL_SAFE_BROWSING" ]
     defines += [ "SAFE_BROWSING_CSD" ]
     defines += [ "SAFE_BROWSING_DB_LOCAL" ]
   } else if (safe_browsing_mode == 2) {
     defines += [ "SAFE_BROWSING_DB_REMOTE" ]
   }
@@ skipping 178 matching lines @@
     # This error doesn't happen every time. In VS2013, it seems if the .pch
     # file doesn't exist, no error will be generated (probably MS tested this
     # case but forgot the other one?). To reproduce this error, do a build,
     # then delete the precompile.c.obj file, then build again.
     cflags_c = [ "/wd4206" ]
   } else if (is_mac && !is_official_build && !use_goma) {
     precompiled_header = "build/precompile.h"
     precompiled_source = "//build/precompile.h"
   }
 }