[Telemetry] Introduce SummarizableValue.

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

 # Copyright 2015 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.
 
 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
 
 
 DEFAULT_METRICS = memory_dump_event.MMAPS_METRICS.keys()
 
 
 def _AggregateDicts(dicts):
   """Turn a sequence of dictionaries into a dictionary of lists."""
   result = collections.defaultdict(list)
@@ skipping 21 matching lines @@
       else:
         metric_values = _AggregateDicts(
             dump.GetMemoryUsage() for dump in memory_dumps)
         none_reason = None
       for metric, values in metric_values.iteritems():
         results.AddValue(list_of_scalar_values.ListOfScalarValues(
             page=results.current_page,
             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())
 
     ReportResultsForProcess(memory_dumps, 'total')
 
     process_dumps_by_name = _AggregateDicts(
         {process_dump.process_name.lower().replace(' ', '_'): process_dump
          for process_dump in memory_dump.IterProcessMemoryDumps()}
         for memory_dump in memory_dumps)
     for process_name, process_dumps in process_dumps_by_name.iteritems():
       ReportResultsForProcess(process_dumps, process_name)