Update the diff_util import in update_histogram_enum.py

tools/metrics/histograms/update_histogram_enum.py

 # 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.
 
 """Updates enums in histograms.xml file with values read from provided C++ enum.
 
 If the file was pretty-printed, the updated version is pretty-printed too.
 """
 
 import logging
+import os
 import print_style
 import re
 import sys
 
 from xml.dom import minidom
-from diffutil import PromptUserToAcceptDiff
+
+sys.path.append(os.path.join(os.path.dirname(__file__), '..', 'common'))
+from diff_util import PromptUserToAcceptDiff
 
 class UserError(Exception):
   def __init__(self, message):
     Exception.__init__(self, message)
 
   @property
   def message(self):
     return self.args[0]
 
 
@@ skipping 100 matching lines @@
   UpdateHistogramDefinitions(histogram_enum_name, source_enum_values,
                              source_enum_path, histograms_doc)
 
   Log('Writing out new histograms file.')
   new_xml = print_style.GetPrintStyle().PrettyPrintNode(histograms_doc)
   if PromptUserToAcceptDiff(xml, new_xml, 'Is the updated version acceptable?'):
     with open(HISTOGRAMS_PATH, 'wb') as f:
       f.write(new_xml)
 
   Log('Done.')