Fix update_histogram_enum.py when UpdateHistogramEnum is called.

tools/metrics/histograms/update_use_counter_feature_enum.py

 #!/usr/bin/env python
 # 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.
 
 """Scans the Chromium source of UseCounter, formats the Feature enum for
 histograms.xml and merges it. This script can also generate a python code
 snippet to put in uma.py of Chromium Dashboard. Make sure that you review the
 output for correctness.
 """
@@ skipping 20 matching lines @@
                     'Chromium dashboard developed at '
                     'https://github.com/GoogleChrome/chromium-dashboard')
   options, args = parser.parse_args()
 
   source_path = 'third_party/WebKit/Source/core/frame/UseCounter.h'
 
   START_MARKER = '^enum Feature : uint32_t {'
   END_MARKER = '^NumberOfFeatures'
 
   if options.dashboard:
-    enum_dict = ReadHistogramValues(source_path, START_MARKER, END_MARKER)
+    enum_dict, ignored = ReadHistogramValues(source_path, START_MARKER,
+        END_MARKER)
     PrintEnumForDashboard(enum_dict)
   else:
     UpdateHistogramEnum(
         histogram_enum_name='FeatureObserver',
         source_enum_path=source_path,
         start_marker=START_MARKER,
         end_marker=END_MARKER)