Reland of Convert enable_pre_sync_backup flag to new flags system.

build/buildflag_header.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.
 
 # Generates a header with preprocessor defines specified by the build file.
 #
 # The canonical documentation is in build/buildflag_header.gni. You should
 # write the GN build, get it working, and then transform it into GYP.
 #
 # In every target that uses your generated header you must include a dependency
@@ skipping 13 matching lines @@
 #   }
 #
 # Write a GYP target like this:
 #
 #  {
 #    # GN version: //foo:foo_features
 #    'target_name': 'foo_foo_features',
 #    'includes': [ '../build/buildflag_header.gypi' ],
 #    'variables': {
 #       'buildflag_header_path': 'foo/foo_features.h',
-#       'buildflag_header_flags': [
+#       'buildflag_flags': [
 #         'ENABLE_DOOM_MELON=<(enable_doom_melon)',
 #         'ENABLE_SPACE_LASER=true',
 #         'SPAM_SERVER_URL="http://www.example.com/"',
 #       ],
 #     },
 #   }
 #
 # Variables
 #
 #   target_name
@@ skipping 64 matching lines @@
       'action': [
         'python', '<(build_header_script)',
         '--output', '<(buildflag_header_path)',
         '--rulename', '<(_target_name)',
         '--gen-dir', '<(SHARED_INTERMEDIATE_DIR)',
         '--definitions', '<(response_file_name)',
       ],
     }
   ],
 }