Try to re-land portion of FailureValue patch (399263003) that should

tools/telemetry/telemetry/results/page_test_results.py

 # 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.
 
 import collections
 import copy
 import logging
 import sys
 import traceback
 
@@ skipping 38 matching lines @@
   def _GetStringFromExcInfo(self, err):
     return ''.join(traceback.format_exception(*err))
 
   def StartTest(self, page):
     pass
 
   def StopTest(self, page):
     pass
 
   def AddValue(self, value):
-    self.ValidateValue(value)
+    self._ValidateValue(value)
     self._all_page_specific_values.append(value)
 
   def AddSummaryValue(self, value):
     assert value.page is None
-    self.ValidateValue(value)
+    self._ValidateValue(value)
     self._all_summary_values.append(value)
 
-  def ValidateValue(self, value):
+  def _ValidateValue(self, value):
     assert isinstance(value, value_module.Value)
     if value.name not in self._representative_value_for_each_value_name:
       self._representative_value_for_each_value_name[value.name] = value
     representative_value = self._representative_value_for_each_value_name[
         value.name]
     assert value.IsMergableWith(representative_value)
 
   def AddFailure(self, page, err):
     self.pages_that_had_failures.add(page)
     self.failures.append((page, self._GetStringFromExcInfo(err)))
@@ skipping 25 matching lines @@
       if value.page == page and value.name == value_name:
         values.append(value)
     return values
 
   def FindAllPageSpecificValuesNamed(self, value_name):
     values = []
     for value in self.all_page_specific_values:
       if value.name == value_name:
         values.append(value)
     return values