Refactor BenchmarkResult.Add to take enum for data_type

tools/telemetry/telemetry/page/page_measurement_value.py

Index: tools/telemetry/telemetry/page/page_measurement_value.py
===================================================================
--- tools/telemetry/telemetry/page/page_measurement_value.py	(revision 217866)
+++ tools/telemetry/telemetry/page/page_measurement_value.py	(working copy)
@@ -1,7 +1,9 @@
 # Copyright (c) 2012 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.
+
 from telemetry.page import perf_tests_helper
+from telemetry.page import result_data_type
 
 def _Mean(l):
   return float(sum(l)) / len(l) if len(l) > 0 else 0.0
@@ -23,7 +25,7 @@
 
   @property
   def output_value(self):
-    if 'histogram' in self.data_type:
+    if result_data_type.IsHistogram(self.data_type):
       (mean, _) = perf_tests_helper.GeomMeanAndStdDevFromHistogram(self.value)
       return mean
     elif isinstance(self.value, list):