headless: Replace is_headless gn setting with an args template

net/BUILD.gn

Index: net/BUILD.gn
diff --git a/net/BUILD.gn b/net/BUILD.gn
index 31aaa3371a54a260095d1d595f72b8759a8122c8..b3ced822762727168114648b836c6984165d7e92 100644
--- a/net/BUILD.gn
+++ b/net/BUILD.gn
@@ -28,10 +28,6 @@ gypi_values = exec_script("//build/gypi_to_gn.py",
                           "scope",
                           [ "net.gypi" ])
 
-# Disable Kerberos on ChromeOS, iOS, Chromecast and headless, at least for now.
-# It needs configuration (krb5.conf and so on).
-use_kerberos = !is_chromeos && !is_ios && !is_chromecast && !is_headless
-
 # The way the cache uses mmap() is inefficient on some Android devices. If
 # this flag is set, we hackily avoid using mmap() in the disk cache. We are
 # pretty confident that mmap-ing the index would not hurt any existing x86
@@ -50,6 +46,10 @@ declare_args() {
   # cronet.
   enable_websockets = !is_ios
   disable_ftp_support = is_ios
+
+  # Disable Kerberos on ChromeOS, iOS, Chromecast, at least for now.
+  # It needs configuration (krb5.conf and so on).
+  use_kerberos = !is_chromeos && !is_ios && !is_chromecast
 }
 
 config("net_config") {