[Mac/GN] In clean_up_old_versions.py, allow the Versions/ directory to not exist.

chrome/tools/build/mac/clean_up_old_versions.py

 # Copyright 2016 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.
 
 import optparse
 import os
 import shutil
 import sys
 
 
 def CleanUpOldVersions(path_to_app, keep_version, stamp_path):
   versions_dir = os.path.join(path_to_app, 'Contents', 'Versions')
+  if not os.path.exists(versions_dir):
+    return
+
   for version in os.listdir(versions_dir):
     if version != keep_version:
       shutil.rmtree(os.path.join(versions_dir, version))
 
   open(stamp_path, 'w').close()
   os.utime(stamp_path, None)
 
 
 def Main():
   parser = optparse.OptionParser(
       usage='%prog path/to/Chromium.app keep_version stamp_path')
   opts, args = parser.parse_args()
   if len(args) != 3:
     parser.error('missing arguments')
     return 1
 
   CleanUpOldVersions(*args)
   return 0
 
 if __name__ == '__main__':
   sys.exit(Main())