[polymer] Merge of master into polymer10-migration

telemetry/telemetry/value/scalar.py

 # Copyright 2013 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.
 
 import numbers
 
 from telemetry import value as value_module
 from telemetry.value import list_of_scalar_values
 from telemetry.value import none_values
 from telemetry.value import summarizable
@@ skipping 98 matching lines @@
     return cls._MergeLikeValues(values, None, v0.name, v0.tir_label,
                                 v0.grouping_keys)
 
   @classmethod
   def _MergeLikeValues(cls, values, page, name, tir_label, grouping_keys):
     v0 = values[0]
     merged_value = [v.value for v in values]
     none_value_reason = None
     if None in merged_value:
       merged_value = None
-      none_value_reason = none_values.MERGE_FAILURE_REASON
+      merged_none_values = [v for v in values if v.value is None]
+      none_value_reason = (
+          none_values.MERGE_FAILURE_REASON +
+          ' None values: %s' % repr(merged_none_values))
     return list_of_scalar_values.ListOfScalarValues(
         page, name, v0.units, merged_value, important=v0.important,
         description=v0.description,
         tir_label=tir_label,
         none_value_reason=none_value_reason,
         improvement_direction=v0.improvement_direction,
         grouping_keys=grouping_keys)