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

tools/perf/metrics/speedindex.py

 # Copyright 2013 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 metrics import Metric
 from telemetry.image_processing import image_util
 from telemetry.image_processing import rgba_color
+from telemetry.value import improvement_direction
 from telemetry.value import scalar
 
 
 class SpeedIndexMetric(Metric):
   """The speed index metric is one way of measuring page load speed.
 
   It is meant to approximate user perception of page load speed, and it
   is based on the amount of time that it takes to paint to the visual
   portion of the screen. It includes paint events that occur after the
   onload event, and it doesn't include time loading things off-screen.
@@ skipping 43 matching lines @@
                     'smaller than the load time. On the other hand, If the '
                     'contents are composed by many XHR requests with small '
                     'main resource and javascript, speed index will be able to '
                     'get the features of performance more accurately than load '
                     'time because the load time will measure the time when '
                     'static resources are loaded. If you want to get more '
                     'detail, please refer to http://goo.gl/Rw3d5d. Currently '
                     'there are two implementations: for Android and for '
                     'Desktop. The Android version uses video capture; the '
                     'Desktop one uses paint events and has extra overhead to '
-                    'catch paint events.'))
+                    'catch paint events.',
+        improvement_direction=improvement_direction.DOWN))
 
   def IsFinished(self, tab):
     """Decide whether the timeline recording should be stopped.
 
     When the timeline recording is stopped determines which paint events
     are used in the speed index metric calculation. In general, the recording
     should continue if there has just been some data received, because
     this suggests that painting may continue.
 
     A page may repeatedly request resources in an infinite loop; a timeout
@@ skipping 233 matching lines @@
     frame = paint_event.args['frameId']
     return (frame,) + GetBox(paint_event.args['data']['clip'])
 
   def _GroupEventByRectangle(self, paint_events):
     """Group all paint events according to the rectangle that they update."""
     result = collections.defaultdict(list)
     for event in paint_events:
       assert event.name == 'Paint'
       result[self._GetRectangle(event)].append(event)
     return result