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

tools/perf/metrics/webrtc_stats.py

Index: tools/perf/metrics/webrtc_stats.py
diff --git a/tools/perf/metrics/webrtc_stats.py b/tools/perf/metrics/webrtc_stats.py
index 4a787dd1afa16d87a8e5a9cda6d7a0cb689ad6ec..b6414105348fc85475a02a829f441c2c4518bf79 100644
--- a/tools/perf/metrics/webrtc_stats.py
+++ b/tools/perf/metrics/webrtc_stats.py
@@ -8,24 +8,29 @@ import re
 
 from metrics import Metric
 from telemetry.core import camel_case
+from telemetry.value import improvement_direction
 from telemetry.value import list_of_scalar_values
 
 INTERESTING_METRICS = {
     'packetsReceived': {
         'units': 'packets',
         'description': 'Packets received by the peer connection',
+        'improvement_direction': improvement_direction.DOWN,
     },
     'packetsSent': {
         'units': 'packets',
         'description': 'Packets sent by the peer connection',
+        'improvement_direction': improvement_direction.DOWN,
     },
     'googDecodeMs': {
         'units': 'ms',
         'description': 'Time spent decoding.',
+        'improvement_direction': improvement_direction.DOWN,
     },
     'googMaxDecodeMs': {
         'units': 'ms',
         'description': 'Maximum time spent decoding one frame.',
+        'improvement_direction': improvement_direction.DOWN,
     },
     # TODO(phoglund): Add much more interesting metrics.
 }
@@ -98,4 +103,6 @@ class WebRtcStatisticsMetric(Metric):
             results.current_page, trace_name,
             INTERESTING_METRICS[general_name]['units'], values,
             description=INTERESTING_METRICS[general_name]['description'],
-            important=False))
+            important=False,
+            improvement_direction=INTERESTING_METRICS[
+                general_name]['improvement_direction']))