Make some chrome feature flags customizable in GN build

chrome/common/features.gni

 # 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.
 
 # This is the GN version of //chrome/chrome_features.gypi.
 # Please keep in sync!
 
 declare_args() {
   enable_google_now = !is_ios && !is_android
 
   # Whether the Java UI is being used (Java infobars and popups, Java native
   # settings and first run experience, sign-in etc.).
   # Default to true if compiling for android, but allow this being overriden
   # through the environment.
   android_java_ui = is_android
 
   # Use vulcanized HTML/CSS/JS resources to speed up WebUI (chrome://)
   # pages. https://github.com/polymer/vulcanize
   use_vulcanize = true
 
+  # Enables support for background apps.
+  enable_background = !is_ios && !is_android
+
+  enable_one_click_signin = is_win || is_mac || (is_linux && !is_chromeos)
+
   # Set to true to bundle all the mash related mojo services into chrome.
   # Specify --mash to chrome to have chrome start the mash environment.
   enable_package_mash_services = is_chromeos
 }
 
-enable_one_click_signin = is_win || is_mac || (is_linux && !is_chromeos)
-
-# Enables support for background apps.
-enable_background = !is_ios && !is_android
-
 chrome_grit_defines = [
   "enable_background=$enable_background",
   "enable_google_now=$enable_google_now",
   "use_vulcanize=$use_vulcanize",
 ]