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

tools/telemetry/telemetry/value/histogram_unittest.py

Index: tools/telemetry/telemetry/value/histogram_unittest.py
diff --git a/tools/telemetry/telemetry/value/histogram_unittest.py b/tools/telemetry/telemetry/value/histogram_unittest.py
index abb0e284a41a7359b74b2a6cbee0722f53feadd3..8513aadfea7e4fb6fb1d21eb2d7e2caf55f3ce65 100644
--- a/tools/telemetry/telemetry/value/histogram_unittest.py
+++ b/tools/telemetry/telemetry/value/histogram_unittest.py
@@ -72,7 +72,8 @@ class ValueTest(TestBase):
       'type': 'histogram',
       'name': 'x',
       'units': 'counts',
-      'buckets': [{'low': 1, 'high': 2, 'count': 1}]
+      'buckets': [{'low': 1, 'high': 2, 'count': 1}],
+      'higher_is_better': True,
     }
     v = value.Value.FromDict(d, {})
 
@@ -80,3 +81,4 @@ class ValueTest(TestBase):
     self.assertEquals(
       ['{"buckets": [{"low": 1, "high": 2, "count": 1}]}'],
       v.GetBuildbotValue())
+    self.assertTrue(v.higher_is_better)