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 2daff4e5b8d21d1788b40a03d59fe1b9b2cd01b5..3a81eb91fe672d6db87bccb9b3d16bce6603fa74 100644
--- a/tools/perf/metrics/speedindex.py
+++ b/tools/perf/metrics/speedindex.py
@@ -69,7 +69,8 @@ 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.',
+        higher_is_better=False))
 
   def IsFinished(self, tab):
     """Decide whether the timeline recording should be stopped.