[chromeos] Add a battery discharge rate plot to about:power page.

chromeos/power/power_data_collector.cc

Index: chromeos/power/power_data_collector.cc
diff --git a/chromeos/power/power_data_collector.cc b/chromeos/power/power_data_collector.cc
index ac22497dcc0b44b104814a3f1e640c7cbb0dc44c..150bc21d9053f7e57f080d2a0e7711f4dc5ba459 100644
--- a/chromeos/power/power_data_collector.cc
+++ b/chromeos/power/power_data_collector.cc
@@ -49,6 +49,7 @@ void PowerDataCollector::PowerChanged(
   snapshot.external_power = (prop.external_power() !=
       power_manager::PowerSupplyProperties::DISCONNECTED);
   snapshot.battery_percent = prop.battery_percent();
+  snapshot.battery_discharge_rate = prop.battery_discharge_rate();
   AddSnapshot(snapshot);
 }