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

tools/perf/measurements/polymer_load.py

Index: tools/perf/measurements/polymer_load.py
diff --git a/tools/perf/measurements/polymer_load.py b/tools/perf/measurements/polymer_load.py
index 9120e31359c23d94bb20e1f1a626729469bae282..397945b2653fb591c8f97b480bf74453da1a3fba 100644
--- a/tools/perf/measurements/polymer_load.py
+++ b/tools/perf/measurements/polymer_load.py
@@ -4,6 +4,7 @@
 
 from telemetry.page import page
 from telemetry.page import page_test
+from telemetry.value import improvement_direction
 from telemetry.value import scalar
 
 
@@ -36,4 +37,5 @@ class PolymerLoadMeasurement(page_test.PageTest):
   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))