[Telemetry] Introduce SummarizableValue.

tools/telemetry/telemetry/web_perf/metrics/memory_timeline.py

Index: tools/telemetry/telemetry/web_perf/metrics/memory_timeline.py
diff --git a/tools/telemetry/telemetry/web_perf/metrics/memory_timeline.py b/tools/telemetry/telemetry/web_perf/metrics/memory_timeline.py
index b691a18c73b2c0f8e541eb8a090230c869adf2d1..c478f486a4f2ac6b51dc4c8927da0e7803df913e 100644
--- a/tools/telemetry/telemetry/web_perf/metrics/memory_timeline.py
+++ b/tools/telemetry/telemetry/web_perf/metrics/memory_timeline.py
@@ -5,6 +5,7 @@
 import collections
 
 from telemetry.timeline import memory_dump_event
+from telemetry.value import improvement_direction
 from telemetry.value import list_of_scalar_values
 from telemetry.web_perf.metrics import timeline_based_metric
 
@@ -46,7 +47,8 @@ class MemoryTimelineMetric(timeline_based_metric.TimelineBasedMetric):
             name='memory_%s_%s' % (metric, process_name),
             units='bytes',
             values=values,
-            none_value_reason=none_reason))
+            none_value_reason=none_reason,
+            improvement_direction=improvement_direction.DOWN))
 
     memory_dumps = filter(HasMmapsDuringInteractions,
                           model.IterGlobalMemoryDumps())