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

tools/metrics/histograms/update_histogram_enum.py

Index: tools/metrics/histograms/update_histogram_enum.py
diff --git a/tools/metrics/histograms/update_histogram_enum.py b/tools/metrics/histograms/update_histogram_enum.py
index db9037ff8bdf614e2976894fc59ae075ef72c814..36a0f4bfa6ab046824535fc4626615c1a5c90c5d 100644
--- a/tools/metrics/histograms/update_histogram_enum.py
+++ b/tools/metrics/histograms/update_histogram_enum.py
@@ -75,13 +75,16 @@ def ReadHistogramValues(filename, start_marker, end_marker):
             label = m.group(1)
           else:
             continue
+        # If two enum constraints have the same value

[Rick Byers, 2017/04/05 15:26:39]

s/constraints/constants/ ?

+        if enum_value in result:
+          return result, (result[enum_value], label)
         result[enum_value] = label
         enum_value += 1
     else:
       if START_REGEX.match(line):
         inside_enum = True
         enum_value = 0
-  return result
+  return result, None
 
 
 def CreateEnumItemNode(document, value, label):
@@ -180,12 +183,14 @@ def HistogramNeedsUpdate(histogram_enum_name, source_enum_path, start_marker,
       end_marker: A regular expression that matches the end of the C++ enum.
   """
   Log('Reading histogram enum definition from "{0}".'.format(source_enum_path))
-  source_enum_values = ReadHistogramValues(source_enum_path, start_marker,
-                                           end_marker)
+  source_enum_values, duplicated_values = ReadHistogramValues(
+      source_enum_path, start_marker, end_marker)
+  if duplicated_values is not None:
+    return False, duplicated_values
 
   (xml, new_xml) = _GetOldAndUpdatedXml(histogram_enum_name, source_enum_values,
                                         source_enum_path)
-  return xml != new_xml
+  return xml != new_xml, None
 
 
 def UpdateHistogramFromDict(histogram_enum_name, source_enum_values,