Readdress use_counter_feature_enum issue brought in

https://codereview.chromium.org/2797043002

Made histogram checker raise exception for duplicated enum values.

BUG=711205

Review-Url: https://codereview.chromium.org/2835733002
Cr-Commit-Position: refs/heads/master@{#469169}

tools/metrics/histograms/update_histogram_enum.py

@@ -33,20 +33,37 @@ def message(self):
     return self.args[0]
 
 
+class DuplicatedValue(Exception):
+  """Exception raised for duplicated enum values.
+
+  Attributes:
+      first_label: First enum label that shares the duplicated enum value.
+      second_label: Second enum label that shares the duplicated enum value.
+  """
+  def __init__(self, first_label, second_label):
+    self.first_label = first_label
+    self.second_label = second_label
+
+
 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.
+  """Creates a dictionary of enum values, read from a C++ file.
 
   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.
+
+  Returns:
+      A boolean indicating wheather the histograms.xml file would be changed.
+
+  Raises:
+      DuplicatedValue: An error when two enum labels share the same value.
   """
   # Read the file as a list of lines
   with open(path_util.GetInputFile(filename)) as f:
@@ -92,10 +109,10 @@ def ReadHistogramValues(filename, start_marker, end_marker, strip_k_prefix):
       label = label[1:]
     # If two enum labels have the same value
     if enum_value in result:
-      return result, (result[enum_value], label)
+      raise DuplicatedValue(result[enum_value], label)
     result[enum_value] = label
     enum_value += 1
-  return result, None
+  return result
 
 
 def CreateEnumItemNode(document, value, label):
@@ -202,14 +219,14 @@ def CheckPresubmitErrors(histogram_enum_name, update_script_name,
       A string with presubmit failure description, or None (if no failures).
   """
   Log('Reading histogram enum definition from "{0}".'.format(source_enum_path))
-  source_enum_values, duplicated_values = ReadHistogramValues(
-      source_enum_path, start_marker, end_marker, strip_k_prefix)
-
-  if duplicated_values:
+  try:
+    source_enum_values = ReadHistogramValues(
+        source_enum_path, start_marker, end_marker, strip_k_prefix)
+  except DuplicatedValue as duplicated_values:
     return ('%s enum has been updated and there exist '
-            'duplicated values between (%s) and (%s)' % (histogram_enum_name,
-                                                         duplicated_values[0],
-                                                         duplicated_values[1]))
+            'duplicated values between (%s) and (%s)' %
+            (histogram_enum_name, duplicated_values.first_label,
+             duplicated_values.second_label))
 
   (xml, new_xml) = _GetOldAndUpdatedXml(histogram_enum_name, source_enum_values,
                                         source_enum_path)