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

tools/telemetry/telemetry/results/page_test_results.py

Index: tools/telemetry/telemetry/results/page_test_results.py
diff --git a/tools/telemetry/telemetry/results/page_test_results.py b/tools/telemetry/telemetry/results/page_test_results.py
index 38b1a668aa2127575a127845fa812d7e8ada4bc4..bd76ce422b07051e7415f03638c579b3d2dd148a 100644
--- a/tools/telemetry/telemetry/results/page_test_results.py
+++ b/tools/telemetry/telemetry/results/page_test_results.py
@@ -56,15 +56,15 @@ class PageTestResults(object):
     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