[GN] Make |use_system_harfbuzz| configurable

third_party/harfbuzz-ng/BUILD.gn

 # Copyright 2014 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.
 
 # The GYP build supports system harfbuzz for non-official builds when using
 # pangoft2 1.31.0 or greater (which pulls it in).
 # TODO(brettw) we can consider doing this as well, although the benefit is
 # unclear and requires shelling out to a script to check the version.
 #
 # ChromeOS uses an up-to-date system one that we have control over, so we
 # don't want to bloat the binary more by including another copy.
-use_system_harfbuzz = is_chromeos
+
+declare_args() {
+  use_system_harfbuzz = is_chromeos
+}
 
 if (use_system_harfbuzz) {
   import("//build/config/linux/pkg_config.gni")
   pkg_config("harfbuzz_pkgconfig") {
     packages = [ "harfbuzz" ]
   }
   group("harfbuzz-ng") {
     direct_dependent_configs = [ ":harfbuzz_pkgconfig" ]
   }
 
@@ skipping 121 matching lines @@
     }
     if (is_mac) {
       sources += [
         "src/hb-coretext.cc",
         "src/hb-coretext.h",
       ]
       defines += [ "HAVE_CORETEXT" ]
     }
   }
 }