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

tools/perf/metrics/speedindex.py

Index: tools/perf/metrics/speedindex.py
diff --git a/tools/perf/metrics/speedindex.py b/tools/perf/metrics/speedindex.py
index 389cd7e0c767565ab1fe4c272d7a6ce31b8e0724..7153fd5ba14a1f46e64bdcfd2b1075872994ecee 100644
--- a/tools/perf/metrics/speedindex.py
+++ b/tools/perf/metrics/speedindex.py
@@ -7,6 +7,7 @@ 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
 
 
@@ -70,7 +71,8 @@ class SpeedIndexMetric(Metric):
                     '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.