Fix error in find annotated tests script

tools/android/find_annotated_tests.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.
 
 """Finds all the annotated tests from proguard dump"""
 
 import argparse
 import datetime
 import json
@@ skipping 144 matching lines @@
                       help='INFO verbosity')
 
   arguments = parser.parse_args(sys.argv[1:])
   logging.basicConfig(
       level=logging.INFO if arguments.verbose else logging.WARNING)
 
   if arguments.timestamp_string is None:
     script_runtime = datetime.datetime.utcnow()
     script_runtime_string = script_runtime.strftime(_EXPORT_TIME_FORMAT)
   else:
-    script_runtime = arguments.timestamp_string
+    script_runtime_string = arguments.timestamp_string
   logging.info('Build time is %s', script_runtime_string)
   apk_output_dir = os.path.abspath(os.path.join(
       constants.DIR_SOURCE_ROOT, arguments.apk_output_dir))
   report_data = _GetReport(
       arguments.test_apks, script_runtime_string, apk_output_dir)
 
   json_output_path = os.path.join(
       arguments.json_output_dir,
       '%s-android-chrome.json' % script_runtime_string)
   with open(json_output_path, 'w') as f:
     json.dump(report_data, f, sort_keys=True, separators=(',',': '))
     logging.info('Saved json output file to %s', json_output_path)
 
 
 if __name__ == '__main__':
   sys.exit(main())