headless: Replace is_headless gn setting with an args template

build/args/headless.gn

+# GN args template for the Headless Chrome library
+#
+# Add import to arg.gn in out directory and run gn gen on the directory to use.
+# E.g. for out directory out/foo:
+# echo 'import("//build/args/headless.gn")' > out/foo/args.gn
+# gn gen out/foo
+#
+# Use gn args to add your own build preference args.
+
+use_ozone = true
+
+enable_basic_printing = false
+enable_print_preview = false
+enable_remoting = false
+use_alsa = false
+use_ash = false
+use_cups = false
+use_dbus = false
+use_gconf = false
+use_gio = false
+use_kerberos = false
+use_pulseaudio = false
+use_udev = false