Refactor BenchmarkResult.Add to take enum for data_type

tools/telemetry/telemetry/page/page_measurement_results.py

Index: tools/telemetry/telemetry/page/page_measurement_results.py
===================================================================
--- tools/telemetry/telemetry/page/page_measurement_results.py	(revision 217866)
+++ tools/telemetry/telemetry/page/page_measurement_results.py	(working copy)
@@ -4,6 +4,7 @@
 
 from telemetry.page import page_test_results
 from telemetry.page import page_measurement_value
+from telemetry.page import result_data_type
 
 class ValuesForSinglePage(object):
   def __init__(self, page):
@@ -72,13 +73,14 @@
   def all_measurements_that_have_been_seen(self):
     return self._all_measurements_that_have_been_seen
 
-  def Add(self, trace_name, units, value, chart_name=None, data_type='default'):
+  def Add(self, trace_name, units, value, chart_name=None,
+          data_type=result_data_type.DEFAULT):
     value = self._GetPageMeasurementValue(trace_name, units, value, chart_name,
                                         data_type)
     self._values_for_current_page.AddValue(value)
 
   def AddSummary(self, trace_name, units, value, chart_name=None,
-                 data_type='default'):
+                 data_type=result_data_type.DEFAULT):
     value = self._GetPageMeasurementValue(trace_name, units, value, chart_name,
                                         data_type)
     self._overall_results.append(value)
@@ -91,6 +93,8 @@
 
     # Sanity checks.
     assert measurement_name != 'url', 'The name url cannot be used'
+    assert result_data_type.IsValidType(data_type), \
+          'Data type must be a valid result_data_type'
     if measurement_name in self._all_measurements_that_have_been_seen:
       measurement_data = \
           self._all_measurements_that_have_been_seen[measurement_name]