Standardize usage of virtual/override/final in base/

base/power_monitor/power_monitor_device_source.h

Index: base/power_monitor/power_monitor_device_source.h
diff --git a/base/power_monitor/power_monitor_device_source.h b/base/power_monitor/power_monitor_device_source.h
index 3d264b444b005ace3dab31628c2b083c838caed0..29f17c2a7d96b95903f43d8be78da5554068bd1b 100644
--- a/base/power_monitor/power_monitor_device_source.h
+++ b/base/power_monitor/power_monitor_device_source.h
@@ -37,7 +37,7 @@ namespace base {
 class BASE_EXPORT PowerMonitorDeviceSource : public PowerMonitorSource {
  public:
   PowerMonitorDeviceSource();
-  virtual ~PowerMonitorDeviceSource();
+  ~PowerMonitorDeviceSource() override;
 
 #if defined(OS_MACOSX)
   // Allocate system resources needed by the PowerMonitor class.
@@ -90,7 +90,7 @@ class BASE_EXPORT PowerMonitorDeviceSource : public PowerMonitorSource {
   // Platform-specific method to check whether the system is currently
   // running on battery power.  Returns true if running on batteries,
   // false otherwise.
-  virtual bool IsOnBatteryPowerImpl() override;
+  bool IsOnBatteryPowerImpl() override;
 
   // Checks the battery status and notifies observers if the battery
   // status has changed.