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

tools/perf/measurements/image_decoding.py

 # Copyright 2012 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 from metrics import power
 
 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
 
 
 class ImageDecoding(page_test.PageTest):
   def __init__(self):
     super(ImageDecoding, self).__init__(
       action_name_to_run='RunPageInteractions')
     self._power_metric = None
 
   def CustomizeBrowserOptions(self, options):
@@ skipping 66 matching lines @@
       decode_image_events = decode_image_events[-min_iterations:]
 
     durations = [d.duration for d in decode_image_events]
     assert durations, 'Failed to find image decode trace events.'
 
     image_decoding_avg = sum(durations) / len(durations)
     results.AddValue(scalar.ScalarValue(
         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
     if tracing_controller.is_tracing_running:
       tracing_controller.Stop()