Remove obsolete fieldtrial related code.

tools/metrics/histograms/pretty_print.py

 #!/usr/bin/env python
 # Copyright 2013 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.
 
 """Pretty-prints the histograms.xml file, alphabetizing tags, wrapping text
 at 80 chars, enforcing standard attribute ordering, and standardizing
 indentation.
 
 This is quite a bit more complicated than just calling tree.toprettyxml();
@@ skipping 22 matching lines @@
 # and the value is a pair of the tag name of the children we want to sort,
 # and a key function that maps each child node to the desired sort key.
 ALPHABETIZATION_RULES = {
   'histograms': ('histogram', lambda n: n.attributes['name'].value.lower()),
   'enums': ('enum', lambda n: n.attributes['name'].value.lower()),
   'enum': ('int', lambda n: int(n.attributes['value'].value)),
   'histogram_suffixes_list': (
       'histogram_suffixes', lambda n: n.attributes['name'].value.lower()),
   'histogram_suffixes': ('affected-histogram',
                          lambda n: n.attributes['name'].value.lower()),
-  # TODO(yiyaoliu): Remove fieldtrial related pieces when it is not used.
-  'fieldtrials': ('fieldtrial', lambda n: n.attributes['name'].value.lower()),
-  'fieldtrial': ('affected-histogram',
-                 lambda n: n.attributes['name'].value.lower()),
 }
 
 
 class Error(Exception):
   pass
 
 
 def unsafeAppendChild(parent, child):
   """Append child to parent's list of children, ignoring the possibility that it
   is already in another node's childNodes list.  Requires that the previous
@@ skipping 116 matching lines @@
   logging.info('Creating backup file %s' % histograms_backup_filename)
   shutil.move(histograms_pathname, histograms_backup_pathname)
 
   logging.info('Writing new %s file' % histograms_filename)
   with open(histograms_pathname, 'wb') as f:
     f.write(pretty)
 
 
 if __name__ == '__main__':
   main()