Don't explode if --tools or --extra-tools isn't specified.

tools/clang/scripts/update.py

 #!/usr/bin/env python
 # Copyright (c) 2012 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.
 
 """This script is used to download prebuilt clang binaries.
 
 It is also used by package.py to build the prebuilt clang binaries."""
 
 import argparse
@@ skipping 829 matching lines @@
                       help='build LLVM Gold plugin with LTO')
   parser.add_argument('--llvm-force-head-revision', action='store_true',
                       help=('use the revision in the repo when printing '
                             'the revision'))
   parser.add_argument('--print-revision', action='store_true',
                       help='print current clang revision and exit.')
   parser.add_argument('--print-clang-version', action='store_true',
                       help='print current clang version (e.g. x.y.z) and exit.')
   parser.add_argument('--run-tests', action='store_true',
                       help='run tests after building; only for local builds')
-  parser.add_argument('--extra-tools', '--tools', nargs='*',
+  parser.add_argument('--extra-tools', '--tools', nargs='*', default=[],
                       help='select additional chrome tools to build')
   parser.add_argument('--without-android', action='store_false',
                       help='don\'t build Android ASan runtime (linux only)',
                       dest='with_android',
                       default=sys.platform.startswith('linux'))
   args = parser.parse_args()
 
   if args.lto_gold_plugin and not args.bootstrap:
     print '--lto-gold-plugin requires --bootstrap'
     return 1
@@ skipping 46 matching lines @@
     args.force_local_build = True
     if 'OS=android' not in os.environ.get('GYP_DEFINES', ''):
       # Only build the Android ASan rt on ToT bots when targetting Android.
       args.with_android = False
 
   return UpdateClang(args)
 
 
 if __name__ == '__main__':
   sys.exit(main())