Make some chrome feature flags customizable in GN build

chrome/common/features.gni

Index: chrome/common/features.gni
diff --git a/chrome/common/features.gni b/chrome/common/features.gni
index 06e0804209f715e646d8ec72955683bc89a446cc..5cc2a0997b899ba60691f9c597fd06697776e53f 100644
--- a/chrome/common/features.gni
+++ b/chrome/common/features.gni
@@ -18,16 +18,16 @@ declare_args() {
   # 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",