Refactor BenchmarkResult.Add to take enum for data_type

tools/telemetry/telemetry/page/csv_page_measurement_results_unittest.py

Index: tools/telemetry/telemetry/page/csv_page_measurement_results_unittest.py
===================================================================
--- tools/telemetry/telemetry/page/csv_page_measurement_results_unittest.py	(revision 217866)
+++ tools/telemetry/telemetry/page/csv_page_measurement_results_unittest.py	(working copy)
@@ -8,6 +8,7 @@
 
 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({
@@ -107,13 +108,13 @@
     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()