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

tools/perf/benchmarks/robohornet_pro.py

Index: tools/perf/benchmarks/robohornet_pro.py
diff --git a/tools/perf/benchmarks/robohornet_pro.py b/tools/perf/benchmarks/robohornet_pro.py
index d990f9ff69c62c03f4345f735fd51a4688565c6e..54b2e0316ed516268fe2eeef5b385c23af489fdd 100644
--- a/tools/perf/benchmarks/robohornet_pro.py
+++ b/tools/perf/benchmarks/robohornet_pro.py
@@ -11,6 +11,7 @@ from telemetry import benchmark
 from telemetry import page as page_module
 from telemetry.page import page_set
 from telemetry.page import page_test
+from telemetry.value import improvement_direction
 from telemetry.value import scalar
 
 
@@ -40,7 +41,8 @@ class _RobohornetProMeasurement(page_test.PageTest):
 
     result = int(tab.EvaluateJavaScript('stopTime - startTime'))
     results.AddValue(
-        scalar.ScalarValue(results.current_page, 'Total', 'ms', result))
+        scalar.ScalarValue(results.current_page, 'Total', 'ms', result,
+                           improvement_direction=improvement_direction.DOWN))
 
 
 # We plan to remove this test because it doesn't give useful data, but