Convert enable_pre_sync_backup flag to new flags system.

This flag's usage is very isolated so is a good candidate for the new system. There is now a generated header for the #define rather than a global define.

Reland of http://crrev.com/1478623004#ps60001
TBR=zea@chromium.org

Review URL: https://codereview.chromium.org/1487873003

Cr-Original-Commit-Position: refs/heads/master@{#362365}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: db7f9756dc6e236589b90c76d6fa0f1244601e78

buildflag_header.gypi

@@ -31,7 +31,7 @@
 #    '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/"',

common.gypi

@@ -538,9 +538,6 @@
       # Whether one-click signin is enabled or not.
       'enable_one_click_signin%': 0,
 
-      # Whether to back up data before sync.
-      'enable_pre_sync_backup%': 0,
-
       # Enable Chrome browser extensions
       'enable_extensions%': 1,
 
@@ -812,7 +809,6 @@
 
         ['OS=="win" or OS=="mac" or (OS=="linux" and chromeos==0)', {
           'enable_one_click_signin%': 1,
-          'enable_pre_sync_backup%': 1,
         }],
 
         ['OS=="android"', {
@@ -1163,7 +1159,6 @@
     'use_titlecase_in_grd%': '<(use_titlecase_in_grd)',
     'remoting%': '<(remoting)',
     'enable_one_click_signin%': '<(enable_one_click_signin)',
-    'enable_pre_sync_backup%': '<(enable_pre_sync_backup)',
     'enable_media_router%': '<(enable_media_router)',
     'enable_webrtc%': '<(enable_webrtc)',
     'chromium_win_pch%': '<(chromium_win_pch)',
@@ -2735,9 +2730,6 @@
       ['enable_one_click_signin==1', {
         'defines': ['ENABLE_ONE_CLICK_SIGNIN'],
       }],
-      ['enable_pre_sync_backup==1', {
-        'defines': ['ENABLE_PRE_SYNC_BACKUP'],
-      }],
       ['image_loader_extension==1', {
         'defines': ['IMAGE_LOADER_EXTENSION=1'],
       }],

config/BUILD.gn

@@ -239,9 +239,6 @@ config("feature_flags") {
   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" ]
   }

config/features.gni

@@ -191,9 +191,6 @@ use_brlapi = is_chromeos
 # system).
 use_gconf = is_linux && !is_chromeos && !is_chromecast && !is_headless
 
-# Whether to back up data before sync.
-enable_pre_sync_backup = is_win || is_mac || (is_linux && !is_chromeos)
-
 # Enable WebVR support by default on Android
 # Still requires command line flag to access API
 enable_webvr = is_android