Minor fix UpdateHistogramEnum

In https://codereview.chromium.org/2835733002 the ReadHistogramValues
method now returns one value instead of 2, but its call in
UpdateHistogramEnum still expect 2 where the 2nd was ignored.

BUG=711205

Review-Url: https://codereview.chromium.org/2866573002
Cr-Commit-Position: refs/heads/master@{#469532}

tools/metrics/histograms/update_histogram_enum.py

@@ -274,7 +274,7 @@ def UpdateHistogramEnum(histogram_enum_name, source_enum_path,
   """
 
   Log('Reading histogram enum definition from "{0}".'.format(source_enum_path))
-  source_enum_values, ignored = ReadHistogramValues(source_enum_path,
+  source_enum_values = ReadHistogramValues(source_enum_path,
       start_marker, end_marker, strip_k_prefix)
 
   UpdateHistogramFromDict(histogram_enum_name, source_enum_values,