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

tools/perf/measurements/image_decoding.py

Index: tools/perf/measurements/image_decoding.py
diff --git a/tools/perf/measurements/image_decoding.py b/tools/perf/measurements/image_decoding.py
index eed4ce923b23e7aa339d4011d886cc8433e667b0..471a2c0e55c9463c883cfd8d18057835a010dec0 100644
--- a/tools/perf/measurements/image_decoding.py
+++ b/tools/perf/measurements/image_decoding.py
@@ -8,6 +8,7 @@ from telemetry.core.platform import tracing_category_filter
 from telemetry.core.platform import tracing_options
 from telemetry.page import page_test
 from telemetry.timeline import model
+from telemetry.value import improvement_direction
 from telemetry.value import scalar
 
 
@@ -94,10 +95,12 @@ class ImageDecoding(page_test.PageTest):
         results.current_page, 'ImageDecoding_avg', 'ms', image_decoding_avg,
         description='Average decode time for images in 4 different '
                     'formats: gif, png, jpg, and webp. The image files are '
-                    'located at chrome/test/data/image_decoding.'))
+                    'located at chrome/test/data/image_decoding.',
+        improvement_direction=improvement_direction.DOWN))
     results.AddValue(scalar.ScalarValue(
         results.current_page, 'ImageLoading_avg', 'ms',
-        tab.EvaluateJavaScript('averageLoadingTimeMs()')))
+        tab.EvaluateJavaScript('averageLoadingTimeMs()'),
+        improvement_direction=improvement_direction.DOWN))
 
   def CleanUpAfterPage(self, page, tab):
     tracing_controller = tab.browser.platform.tracing_controller