Refactor BenchmarkResult.Add to take enum for data_type

tools/telemetry/telemetry/page/buildbot_page_measurement_results_unittest.py

Index: tools/telemetry/telemetry/page/buildbot_page_measurement_results_unittest.py
===================================================================
--- tools/telemetry/telemetry/page/buildbot_page_measurement_results_unittest.py	(revision 217866)
+++ tools/telemetry/telemetry/page/buildbot_page_measurement_results_unittest.py	(working copy)
@@ -7,6 +7,7 @@
 from telemetry.page import buildbot_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({
@@ -247,15 +248,15 @@
 
     measurement_results = SummarySavingPageMeasurementResults()
     measurement_results.WillMeasurePage(test_page_set.pages[0])
-    measurement_results.Add('a', '',
-                          '{"buckets": [{"low": 1, "high": 2, "count": 1}]}',
-                          data_type='histogram')
+    measurement_results.Add(
+        'a', '', '{"buckets": [{"low": 1, "high": 2, "count": 1}]}',
+        data_type=result_data_type.HISTOGRAM)
     measurement_results.DidMeasurePage()
 
     measurement_results.WillMeasurePage(test_page_set.pages[1])
-    measurement_results.Add('a', '',
-                          '{"buckets": [{"low": 2, "high": 3, "count": 1}]}',
-                          data_type='histogram')
+    measurement_results.Add(
+        'a', '', '{"buckets": [{"low": 2, "high": 3, "count": 1}]}',
+        data_type=result_data_type.HISTOGRAM)
     measurement_results.DidMeasurePage()
 
     measurement_results.PrintSummary()