Add support for customizing GN's args text.

tools/gn/build_settings.cc

Index: tools/gn/build_settings.cc
diff --git a/tools/gn/build_settings.cc b/tools/gn/build_settings.cc
index 3fac976202ad1f5125d2485826e5ec99c8a471a3..1a02bfad4c93b06da19bd2abef0b41eb19ca78d9 100644
--- a/tools/gn/build_settings.cc
+++ b/tools/gn/build_settings.cc
@@ -18,6 +18,7 @@ BuildSettings::BuildSettings(const BuildSettings& other)
       secondary_source_path_(other.secondary_source_path_),
       python_path_(other.python_path_),
       build_config_file_(other.build_config_file_),
+      arg_file_template_path_(other.arg_file_template_path_),
       build_dir_(other.build_dir_),
       build_args_(other.build_args_) {
 }