Address feedback

cobalt/build/gn.py

@@ -103,18 +103,15 @@ def main(out_directory: str, platform: str, build_type: str,
   Path(out_directory).mkdir(parents=True, exist_ok=True)
 
   build_args = get_build_args(src_args_gn_file)
-  if not os.path.exists(dst_args_gn_file):
-    write_build_args(dst_args_gn_file, build_args[0], build_args[1], build_type)
-  else:
-    # Overwrite args.gn, copy the stale args.gn file to stale_args.gn
+  if os.path.exists(dst_args_gn_file):
+    # Copy the stale args.gn into stale_args.gn
     stale_dst_args_gn_file = dst_args_gn_file.replace('args', 'stale_args')
     os.rename(dst_args_gn_file, stale_dst_args_gn_file)
     print(f' Warning: {dst_args_gn_file} is rewritten.' +
           ' Old file is copied to {stale_dst_args_gn_file}.' +
           'In general, if the file exists, you should run' +
           ' `gn args <out_directory>` to edit it instead.')
-    write_build_args(dst_args_gn_file, build_args[0], build_args[1], build_type)
-
+  write_build_args(dst_args_gn_file, build_args[0], build_args[1], build_type)
   gn_command = ['gn', 'gen', out_directory] + gn_gen_args
   print(' '.join(gn_command))
   subprocess.check_call(gn_command)