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

tools/perf/metrics/startup_metric.py

Index: tools/perf/metrics/startup_metric.py
diff --git a/tools/perf/metrics/startup_metric.py b/tools/perf/metrics/startup_metric.py
index abf62da3bdb9234f614285f397d6264c795f647c..94d097057e22f8a5fb7a404c0ff294ee3ebfad2f 100644
--- a/tools/perf/metrics/startup_metric.py
+++ b/tools/perf/metrics/startup_metric.py
@@ -9,6 +9,7 @@ from metrics import Metric
 
 from telemetry.core import util
 from telemetry.value import histogram_util
+from telemetry.value import improvement_direction
 from telemetry.value import scalar
 
 
@@ -93,11 +94,13 @@ class StartupMetric(Metric):
         foreground_tab_stats.load_duration_ms) - browser_main_entry_time_ms)
     results.AddValue(scalar.ScalarValue(
         results.current_page, 'foreground_tab_load_complete', 'ms',
-        foreground_tab_load_complete))
+        foreground_tab_load_complete,
+        improvement_direction=improvement_direction.DOWN))
     if (foreground_tab_stats.request_start_ms > 0):
       results.AddValue(scalar.ScalarValue(
           results.current_page, 'foreground_tab_request_start', 'ms',
-          foreground_tab_stats.request_start_ms - browser_main_entry_time_ms))
+          foreground_tab_stats.request_start_ms - browser_main_entry_time_ms,
+          improvement_direction=improvement_direction.DOWN))
 
   def AddResults(self, tab, results):
     get_histogram_js = 'statsCollectionController.getBrowserHistogram("%s")'
@@ -116,7 +119,8 @@ class StartupMetric(Metric):
             (result['buckets'][0]['high'] + result['buckets'][0]['low']) / 2
 
       results.AddValue(scalar.ScalarValue(
-          results.current_page, display_name, 'ms', measured_time))
+          results.current_page, display_name, 'ms', measured_time,
+          improvement_direction=improvement_direction.DOWN))
 
     # Get tab load times.
     browser_main_entry_time_ms = self._GetBrowserMainEntryTime(tab)