Implement base::PowerMonitor::IsOnBatteryPower() for OSX.

base/power_monitor/power_monitor_source.h

Index: base/power_monitor/power_monitor_source.h
diff --git a/base/power_monitor/power_monitor_source.h b/base/power_monitor/power_monitor_source.h
index e63f4f82bf959fc8d134203d9242fe32564ce793..b69cbf8317d76c8134c0dd237e5dd0017c53e0ce 100644
--- a/base/power_monitor/power_monitor_source.h
+++ b/base/power_monitor/power_monitor_source.h
@@ -49,9 +49,14 @@ class BASE_EXPORT PowerMonitorSource {
   // false otherwise.
   virtual bool IsOnBatteryPowerImpl() = 0;
 
+  // Sets the initial state for |on_battery_power_|, which defaults to false
+  // since not all implementations can provide the value at construction. May
+  // only be called before a base::PowerMonitor has been created.
+  void SetInitialOnBatteryPowerState(bool on_battery_power);
+
  private:
-  bool on_battery_power_;
-  bool suspended_;
+  bool on_battery_power_ = false;
+  bool suspended_ = false;
 
   // This lock guards access to on_battery_power_, to ensure that
   // IsOnBatteryPower can be called from any thread.