Refactor BenchmarkResult.Add to take enum for data_type

tools/perf/metrics/media.py

Index: tools/perf/metrics/media.py
===================================================================
--- tools/perf/metrics/media.py	(revision 217866)
+++ tools/perf/metrics/media.py	(working copy)
@@ -5,6 +5,7 @@
 import os
 
 from metrics import Metric
+from telemetry.page import result_data_type
 
 class MediaMetric(Metric):
   """MediaMetric class injects and calls JS responsible for recording metrics.
@@ -52,7 +53,8 @@
         if m.startswith(metric):
           special_label = m[len(metric):]
           results.Add(trace + special_label, unit, str(metrics[m]),
-                      chart_name=metric, data_type='default')
+                      chart_name=metric,
+                      data_type=result_data_type.DEFAULT)
 
     trace = media_metric['id']
     if not trace: