Refactor BenchmarkResult.Add to take enum for data_type

tools/telemetry/telemetry/page/page_measurement_results_unittest.py

Index: tools/telemetry/telemetry/page/page_measurement_results_unittest.py
===================================================================
--- tools/telemetry/telemetry/page/page_measurement_results_unittest.py	(revision 217866)
+++ tools/telemetry/telemetry/page/page_measurement_results_unittest.py	(working copy)
@@ -7,6 +7,7 @@
 from telemetry.page import page_measurement_results
 from telemetry.page import page_set
 from telemetry.page import perf_tests_helper
+from telemetry.page import result_data_type
 
 def _MakePageSet():
   return page_set.PageSet.FromDict({
@@ -84,7 +85,9 @@
     measurement_results.WillMeasurePage(test_page_set.pages[1])
     self.assertRaises(
       AssertionError,
-      lambda: measurement_results.Add('a', 'seconds', 3, data_type='histogram'))
+      lambda: measurement_results.Add(
+          'a', 'seconds', 3,
+          data_type=result_data_type.HISTOGRAM))
 
   def test_basic_summary_all_pages_fail(self):
     """If all pages fail, no summary is printed."""