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

chromeos/power/power_data_collector.h

Index: chromeos/power/power_data_collector.h
diff --git a/chromeos/power/power_data_collector.h b/chromeos/power/power_data_collector.h
index 49da54511b93a7610757fdcb88254b5ceb3ccd95..cf4af72300e149a267721b16a844b3938b007658 100644
--- a/chromeos/power/power_data_collector.h
+++ b/chromeos/power/power_data_collector.h
@@ -38,6 +38,10 @@ class CHROMEOS_EXPORT PowerDataCollector : public PowerManagerClient::Observer {
 
     // The battery charge as a percentage of full charge in range [0.0, 100.00].
     double battery_percent;
+
+    // The battery discharge rate in W. Positive if the battery is being
+    // discharged and negative if it's being charged.
+    double battery_discharge_rate;
   };
 
   const std::deque<PowerSupplySnapshot>& power_supply_data() const {