Convert enable_pre_sync_backup flag to new flags system.

components/sync_driver/BUILD.gn

Index: components/sync_driver/BUILD.gn
diff --git a/components/sync_driver/BUILD.gn b/components/sync_driver/BUILD.gn
index ffe41b8b23f1dc137cda814fbe564932764f8486..38b89a8c3e2747c69eb7901f849d95ba69b565a6 100644
--- a/components/sync_driver/BUILD.gn
+++ b/components/sync_driver/BUILD.gn
@@ -2,6 +2,7 @@
 # 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") {
@@ -120,6 +121,9 @@ source_set("sync_driver") {
     "user_selectable_sync_type.h",
   ]
 
+  public_deps = [
+    ":features",
+  ]
   deps = [
     "//base",
     "//base:prefs",
@@ -250,3 +254,12 @@ source_set("unit_tests") {
     ]
   }
 }
+
+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" ]
+}