Added support for improvement_direction to relevant values, which is propogated to chartjson.

tools/telemetry/telemetry/web_perf/metrics/layout.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.
 
 import math
+from telemetry.value import improvement_direction
 from telemetry.value import scalar
 from telemetry.web_perf.metrics import timeline_based_metric
 
 
 class LayoutMetric(timeline_based_metric.TimelineBasedMetric):
   """Computes metrics that measure layout performance, specifically,
   avg and stddev of CPU time of layout-related trace events:
 
     layout_total_{avg,stddev}: FrameView::layout
     layout_{avg,stddev}: FrameView::performLayout
@@ skipping 28 matching lines @@
 
     for long_name, (short_name, durations) in metrics.iteritems():
       count = len(durations)
       avg = 0.0
       stddev = 0.0
       if count:
         avg = sum(durations) / count
         stddev = math.sqrt(sum((d - avg) ** 2 for d in durations) / count)
       results.AddValue(scalar.ScalarValue(results.current_page,
           short_name + '_avg', 'ms', avg,
-          description='Average duration of %s events' % long_name))
+          description='Average duration of %s events' % long_name,
+          improvement_direction=improvement_direction.DOWN))
       results.AddValue(scalar.ScalarValue(results.current_page,
           short_name + '_stddev', 'ms', stddev,
-          description='stddev of duration of %s events' % long_name))
+          description='stddev of duration of %s events' % long_name,
+          improvement_direction=improvement_direction.DOWN))