Refactor BenchmarkResult.Add to take enum for data_type

tools/telemetry/telemetry/page/csv_page_measurement_results_unittest.py

 # 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.
 import StringIO
 import csv
 import os
 import unittest
 
 from telemetry.page import csv_page_measurement_results
 from telemetry.page import page_set
+from telemetry.page import result_data_type
 
 def _MakePageSet():
   return page_set.PageSet.FromDict({
       "description": "hello",
       "pages": [
         {"url": "http://www.foo.com/"},
         {"url": "http://www.bar.com/"}
         ]
       }, os.path.dirname(__file__))
 
@@ skipping 79 matching lines @@
     self.assertEquals(
       self.output_data_rows,
       [[self._page_set[0].url, '-', '3'],
        [self._page_set[1].url, '4', '-']])
 
   def test_histogram(self):
     results = NonPrintingCsvPageMeasurementResults(self._output, False)
     results.WillMeasurePage(self._page_set[0])
     results.Add('a', '',
                 '{"buckets": [{"low": 1, "high": 2, "count": 1}]}',
-                data_type='histogram')
+                data_type=result_data_type.HISTOGRAM)
     results.DidMeasurePage()
 
     results.WillMeasurePage(self._page_set[1])
     results.Add('a', '',
                 '{"buckets": [{"low": 2, "high": 3, "count": 1}]}',
-                data_type='histogram')
+                data_type=result_data_type.HISTOGRAM)
     results.DidMeasurePage()
 
     results.PrintSummary()
 
     self.assertEquals(
       self.output_header_row,
       ['url', 'a ()'])
     self.assertEquals(
       self.output_data_rows,
       [[self._page_set[0].url, '1.5'],
        [self._page_set[1].url, '2.5']])