Histograms.xml python script housekeeping

tools/metrics/histograms/update_extension_functions.py

 # 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.
 
 """Updates ExtensionFunctions enum in histograms.xml file with values read from
 extension_function_histogram_value.h.
 
 If the file was pretty-printed, the updated version is pretty-printed too.
 """
 
 import os
 import sys
 
 from update_histogram_enum import UpdateHistogramEnum
 
 if __name__ == '__main__':
   if len(sys.argv) > 1:
     print >>sys.stderr, 'No arguments expected!'
     sys.stderr.write(__doc__)
     sys.exit(1)
 
+  source_header = 'extensions/browser/extension_function_histogram_value.h'
   UpdateHistogramEnum(histogram_enum_name='ExtensionFunctions',
-                      source_enum_path=
-                          os.path.join('..', '..', '..', 'extensions',
-                                       'browser',
-                                       'extension_function_histogram_value.h'),
+                      source_enum_path=source_header,
                       start_marker='^enum HistogramValue {',
                       end_marker='^ENUM_BOUNDARY')