Standardize usage of virtual/override/final specifiers in base/.

base/power_monitor/power_monitor_unittest.cc

Index: base/power_monitor/power_monitor_unittest.cc
diff --git a/base/power_monitor/power_monitor_unittest.cc b/base/power_monitor/power_monitor_unittest.cc
index 5f7b531206f2adcea05f91ffea8db81782f8a9c1..2df82618f4ccee853a8ffaabefcd65b26cda757f 100644
--- a/base/power_monitor/power_monitor_unittest.cc
+++ b/base/power_monitor/power_monitor_unittest.cc
@@ -15,7 +15,7 @@ class PowerMonitorTest : public testing::Test {
     power_monitor_.reset(new PowerMonitor(
         scoped_ptr<PowerMonitorSource>(power_monitor_source_)));
   }
-  virtual ~PowerMonitorTest() {};
+  ~PowerMonitorTest() override{};
 
   PowerMonitorTestSource* source() { return power_monitor_source_; }
   PowerMonitor* monitor() { return power_monitor_.get(); }