Add BUG=None to autoroll CLs for chromium_revision in DEPS.

tools/autoroller/roll_chromium_revision.py

 #!/usr/bin/env python
 # Copyright (c) 2015 The WebRTC project authors. All Rights Reserved.
 #
 # Use of this source code is governed by a BSD-style license
 # that can be found in the LICENSE file in the root of the source
 # tree. An additional intellectual property rights grant can be found
 # in the file PATENTS.  All contributing project authors may
 # be found in the AUTHORS file in the root of the source tree.
 
 """Script to roll chromium_revision in the WebRTC DEPS file."""
@@ skipping 273 matching lines @@
   if clang_change.current_rev != clang_change.new_rev:
     commit_msg.append('Clang version changed %s:%s' %
                       (clang_change.current_rev, clang_change.new_rev))
     change_url = CHROMIUM_FILE_TEMPLATE % (rev_interval,
                                            CLANG_UPDATE_SCRIPT_URL_PATH)
     commit_msg.append('Details: %s\n' % change_url)
   else:
     commit_msg.append('No update to Clang.\n')
 
   commit_msg.append('TBR=%s' % tbr_authors)
+  commit_msg.append('BUG=None')
   return '\n'.join(commit_msg)
 
 
 def UpdateDeps(deps_filename, old_cr_revision, new_cr_revision):
   """Update the DEPS file with the new revision."""
   with open(deps_filename, 'rb') as deps_file:
     deps_content = deps_file.read()
   deps_content = deps_content.replace(old_cr_revision, new_cr_revision)
   with open(deps_filename, 'wb') as deps_file:
     deps_file.write(deps_content)
@@ skipping 132 matching lines @@
   _CreateRollBranch(opts.dry_run)
   UpdateDeps(deps_filename, current_cr_rev, new_cr_rev)
   _LocalCommit(commit_msg, opts.dry_run)
   _UploadCL(opts.dry_run, opts.rietveld_email)
   _SendToCQ(opts.dry_run, opts.skip_cq)
   return 0
 
 
 if __name__ == '__main__':
   sys.exit(main())