[Telemetry] Introduce SummarizableValue.

tools/telemetry/telemetry/web_perf/metrics/single_event.py

 # Copyright 2014 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
+from telemetry.value import improvement_direction
 from telemetry.value import list_of_scalar_values
 from telemetry.web_perf.metrics import timeline_based_metric
 
 
 class _SingleEventMetric(timeline_based_metric.TimelineBasedMetric):
   """Reports directly durations of specific trace events that start during the
   user interaction.
   """
 
   def __init__(self, trace_event_name, metric_name, metric_description=None):
@@ skipping 17 matching lines @@
           events_found.append(event.thread_duration)
         else:
           events_found.append(event.duration)
     if not events_found:
       return
     results.AddValue(list_of_scalar_values.ListOfScalarValues(
       page=results.current_page,
       name=self._metric_name,
       units='ms',
       values=events_found,
-      description=self._metric_description))
+      description=self._metric_description,
+      improvement_direction=improvement_direction.DOWN))