Rename ValueSet* to HistogramSet*.

tracing/bin/valueset2html

 #!/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.
 
 import argparse
 import codecs
 import sys
 import os
 
 tracing_path = os.path.abspath(os.path.join(
   os.path.dirname(os.path.realpath(__file__)), '..'))
 sys.path.append(tracing_path)
 from tracing import results_renderer
 
 
 def main():
-  parser = argparse.ArgumentParser(description='Upgrade a results2 instance or '
-                                   'add a new ValueSet JSON.', add_help=False)
+  parser = argparse.ArgumentParser(
+      description='Upgrade a results2 instance or add a new HistogramSet.',
+      add_help=False)
   parser.add_argument('html_path', metavar='HTML_PATH',
                       help='HTML file path (output).')
   parser.add_argument('-h', '--help', action='help',
                       help='Show this help message and exit.')
   args = parser.parse_args()
 
   open(args.html_path, 'a').close() # Create file if it doesn't exist.
   with codecs.open(args.html_path,
                    mode='r+', encoding='utf-8') as output_stream:
     results_renderer.RenderHTMLView([], output_stream)
 
 if __name__ == '__main__':
   sys.exit(main())