Add a presubmit checker for duplicated enum values in UseCounter::Feature

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
@@ skipping 20 matching lines @@
   @property
   def message(self):
     return self.args[0]
 
 
 def Log(message):
   logging.info(message)
 
 
 def ReadHistogramValues(filename, start_marker, end_marker, strip_k_prefix):
-  """Returns a dictionary of enum values and a pair of labels that have the same
-     enum values, read from a C++ file.
+  """Returns a dictionary of enum values, read from a C++ file; or raises an
+     exception if a pair of labels have the same enum values.
 
   Args:
       filename: The unix-style path (relative to src/) of the file to open.
       start_marker: A regex that signifies the start of the enum values.
       end_marker: A regex that signifies the end of the enum values.
       strip_k_prefix: Set to True if enum values are declared as kFoo and the
           'k' should be stripped.
   """
   # Read the file as a list of lines
   with open(path_util.GetInputFile(filename)) as f:
@@ skipping 29 matching lines @@
         next_line = next(iterator).strip()
         enum_value = int(WRAPPED_INIT.match(next_line).group(1))
     else:
       m = ITEM_REGEX.match(line)
       if m:
         label = m.group(1)
       else:
         continue
     # If two enum labels have the same value
     if enum_value in result:
-      return result, (result[enum_value], label)
+      raise Exception(result[enum_value], label)
     if strip_k_prefix:
       assert label.startswith('k'), "Enum " + label + " should start with 'k'."
       label = label[1:]
     result[enum_value] = label
     enum_value += 1
-  return result, None
+  return result
 
 
 def CreateEnumItemNode(document, value, label):
   """Creates an int element to append to an enum."""
   item_node = document.createElement('int')
   item_node.attributes['value'] = str(value)
   item_node.attributes['label'] = label
   return item_node
 
 
@@ skipping 130 matching lines @@
       strip_k_prefix: Set to True if enum values are declared as kFoo and the
           'k' should be stripped.
   """
 
   Log('Reading histogram enum definition from "{0}".'.format(source_enum_path))
   source_enum_values, ignored = ReadHistogramValues(source_enum_path,
       start_marker, end_marker, strip_k_prefix)
 
   UpdateHistogramFromDict(histogram_enum_name, source_enum_values,
       source_enum_path)