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

tools/perf/measurements/tab_switching.py

 # Copyright 2013 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.
 
 """The tab switching measurement.
 
 This measurement opens pages in different tabs. After all the tabs have opened,
 it cycles through each tab in sequence, and records a histogram of the time
 between when a tab was first requested to be shown, and when it was painted.
 Power usage is also measured.
 """
 
 import time
 
 from metrics import power
 from telemetry.core import util
 from telemetry.page import page_test
 from telemetry.value import histogram
 from telemetry.value import histogram_util
+from telemetry.value import improvement_direction
 
 # TODO: Revisit this test once multitab support is finalized.
 
 class TabSwitching(page_test.PageTest):
 
   # Amount of time to measure, in seconds.
   SAMPLE_TIME = 30
 
   def __init__(self):
     super(TabSwitching, self).__init__()
@@ skipping 59 matching lines @@
       util.WaitFor(_IsDone, 30)
       prev_histogram = histogram_util.GetHistogram(
           histogram_type, histogram_name, tab)
 
     last_histogram = histogram_util.GetHistogram(
         histogram_type, histogram_name, tab)
     diff_histogram = histogram_util.SubtractHistogram(last_histogram,
         first_histogram)
 
     results.AddSummaryValue(
-        histogram.HistogramValue(None, display_name, 'ms',
-                                 raw_value_json=diff_histogram,
-                                 important=False))
+        histogram.HistogramValue(
+            None, display_name, 'ms', raw_value_json=diff_histogram,
+            important=False, improvement_direction=improvement_direction.DOWN))