headless: Replace is_headless gn setting with an args template

build/config/ui.gni

Index: build/config/ui.gni
diff --git a/build/config/ui.gni b/build/config/ui.gni
index 1df101f1c850389ab03c94725cd70cee1be6e913..43d53a250f6104562ffea0e0c2c3bb81ff75ad45 100644
--- a/build/config/ui.gni
+++ b/build/config/ui.gni
@@ -15,17 +15,16 @@
 # to set up feature flags.
 
 import("//build/config/chromecast_build.gni")
-import("//build/config/headless_build.gni")
 
 declare_args() {
   # Indicates if Ash is enabled. Ash is the Aura Shell which provides a
   # desktop-like environment for Aura. Requires use_aura = true
-  use_ash = is_chromeos && !is_chromecast && !is_headless
+  use_ash = is_chromeos && !is_chromecast
 
   # Indicates if Ozone is enabled. Ozone is a low-level library layer for Linux
   # that does not require X11. Enabling this feature disables use of glib, x11,
   # Pango, and Cairo. Default to false on non-Chromecast builds.
-  use_ozone = (is_chromecast || is_headless) && !is_android
+  use_ozone = is_chromecast && !is_android
 
   # Indicates if Aura is enabled. Aura is a low-level windowing library, sort
   # of a replacement for GDI or GTK.