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

tools/perf/measurements/screenshot.py

 # Copyright 2014 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.
 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
 
 
 class Screenshot(page_test.PageTest):
   def __init__(self, png_outdir):
     super(Screenshot, self).__init__(
         action_name_to_run = 'RunPageInteractions',
         is_action_name_to_run_optional=True)
     self._png_outdir = png_outdir
 
@@ skipping 13 matching lines @@
     else:
       previous_mtime = -1
 
     image_util.WritePngFile(screenshot, outpath)
 
     saved_picture_count = 0
     if os.path.exists(outpath) and os.path.getmtime(outpath) > previous_mtime:
       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))