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

tools/perf/measurements/polymer_load.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.
 
 from telemetry.page import page
 from telemetry.page import page_test
+from telemetry.value import improvement_direction
 from telemetry.value import scalar
 
 
 class PageForPolymerLoad(page.Page):
 
   def __init__(self, url, page_set, ready_event='polymer-ready'):
     super(PageForPolymerLoad, self).__init__(
       url=url,
       page_set=page_set)
     self.script_to_evaluate_on_commit = '''
@@ skipping 12 matching lines @@
 
 
 class PolymerLoadMeasurement(page_test.PageTest):
   def __init__(self):
     super(PolymerLoadMeasurement, self).__init__(
       action_name_to_run='RunPageInteractions')
 
   def ValidateAndMeasurePage(self, _, tab, results):
     result = int(tab.EvaluateJavaScript('__polymer_ready_time'))
     results.AddValue(scalar.ScalarValue(
-        results.current_page, 'Total', 'ms', result))
+        results.current_page, 'Total', 'ms', result,
+        improvement_direction=improvement_direction.DOWN))