[Telemetry] Introduce SummarizableValue.

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

Index: tools/telemetry/telemetry/web_perf/metrics/single_event.py
diff --git a/tools/telemetry/telemetry/web_perf/metrics/single_event.py b/tools/telemetry/telemetry/web_perf/metrics/single_event.py
index 62c09f1a50c2d9db7c22f0e941d49576d62e47c9..d98b406bcce03e6f47f06f3f1357b097ebcb55c3 100644
--- a/tools/telemetry/telemetry/web_perf/metrics/single_event.py
+++ b/tools/telemetry/telemetry/web_perf/metrics/single_event.py
@@ -2,6 +2,7 @@
 # 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
 
@@ -39,4 +40,5 @@ class _SingleEventMetric(timeline_based_metric.TimelineBasedMetric):
       name=self._metric_name,
       units='ms',
       values=events_found,
-      description=self._metric_description))
+      description=self._metric_description,
+      improvement_direction=improvement_direction.DOWN))