Fix the build script to be able to build more than one generator.

mojom/tools/build_generators.py

 #!/usr/bin/env python
 # 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.
 
 """Builds and optionally uploads the mojom generators to Google Cloud Storage.
 
 This script works the same as build_mojom_tool.
 """
 
 import os
 import sys
 
 import builder
 
 
 def main():
-  parser = builder.get_arg_parser("Build the mojom generators.")
+  all_generators = set(['deps'])
+  parser = builder.get_arg_parser('Build the mojom generators.')
+  parser.add_argument('--generators', dest='generators', type=str,
+      default=all_generators.join(','), action='store',
+      help='Comma-separated list of generators to be built by this build '
+      'script. These should be directories under mojom/generators. By default '
+      'every generator known to the build script will be built.')
   args = parser.parse_args()
 
-  generators = [
-      'deps',
-      ]
+  generators = args.generators.split(',')
+  for generator in generators:
+    if generator not in all_generators:
+      print "The only allowed generators are: %s" % all_generators.join(',')
+      return -1
 
   final_result = 0
   for generator in generators:
     generator_builder = builder.get_builder(
         args=args,
         target_dir='generators',
         binary_name=generator,
         src_path=os.path.join('mojom', 'generators', generator))
 
     if args.upload:
       result = generator_builder.build_and_upload()
     result = generator_builder.build()
     if result != 0:
       final_result = result
       if not args.keep_going:
         return result
 
-    return final_result
+  return final_result
 
 
 if __name__ == '__main__':
   sys.exit(main())