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

tools/perf/measurements/screenshot.py

Index: tools/perf/measurements/screenshot.py
diff --git a/tools/perf/measurements/screenshot.py b/tools/perf/measurements/screenshot.py
index 37936f8a90a80482afc629d3eb589ea3d61ac48f..7b514450a174ebbd04f8201c4f2b2301fa571460 100644
--- a/tools/perf/measurements/screenshot.py
+++ b/tools/perf/measurements/screenshot.py
@@ -6,6 +6,7 @@ import os
 from telemetry.image_processing import image_util
 from telemetry.page import page_test
 from telemetry.page import page_test
+from telemetry.value import improvement_direction
 from telemetry.value import scalar
 
 
@@ -39,4 +40,5 @@ class Screenshot(page_test.PageTest):
       saved_picture_count = 1
     results.AddValue(scalar.ScalarValue(
         results.current_page, 'saved_picture_count', 'count',
-        saved_picture_count))
+        saved_picture_count,
+        improvement_direction=improvement_direction.DOWN))