Add missing source to gn bootstrap.

tools/gn/bootstrap/bootstrap.py

 #!/usr/bin/env python
 # Copyright 2014 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.
 
 """Bootstraps gn.
 
 It is done by first building it manually in a temporary directory, then building
 it with its own BUILD.gn to the final destination.
 """
@@ skipping 192 matching lines @@
       'base/pickle.cc',
       'base/process/kill.cc',
       'base/process/process_iterator.cc',
       'base/process/process_metrics.cc',
       'base/profiler/alternate_timer.cc',
       'base/profiler/tracked_time.cc',
       'base/run_loop.cc',
       'base/sequence_checker_impl.cc',
       'base/sequenced_task_runner.cc',
       'base/sha1_portable.cc',
+      'base/strings/pattern.cc',
       'base/strings/string16.cc',
       'base/strings/string_number_conversions.cc',
       'base/strings/string_piece.cc',
       'base/strings/string_split.cc',
       'base/strings/string_util.cc',
       'base/strings/string_util_constants.cc',
       'base/strings/stringprintf.cc',
       'base/strings/utf_string_conversion_utils.cc',
       'base/strings/utf_string_conversions.cc',
       'base/synchronization/cancellation_flag.cc',
@@ skipping 213 matching lines @@
     cmd.append('-v')
   cmd.append('gn')
   check_call(cmd)
 
   if not options.debug:
     check_call(['strip', os.path.join(build_dir, 'gn')])
 
 
 if __name__ == '__main__':
   sys.exit(main(sys.argv[1:]))