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 f3608532e570cb7e8003f7efddeacd27a1be7f5d..f0080d0215ba5c43db8e7f0c3bdb601c2b475af7 100644
--- a/tools/perf/measurements/image_decoding.py
+++ b/tools/perf/measurements/image_decoding.py
@@ -93,10 +93,12 @@ 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.',
+        higher_is_better=False))
     results.AddValue(scalar.ScalarValue(
         results.current_page, 'ImageLoading_avg', 'ms',
-        tab.EvaluateJavaScript('averageLoadingTimeMs()')))
+        tab.EvaluateJavaScript('averageLoadingTimeMs()'),
+        higher_is_better=False))
 
   def CleanUpAfterPage(self, page, tab):
     tracing_controller = tab.browser.platform.tracing_controller