Standardize usage of virtual/override/final in base/

base/metrics/stats_counters.h

Index: base/metrics/stats_counters.h
diff --git a/base/metrics/stats_counters.h b/base/metrics/stats_counters.h
index a2c7decb660a5c7ac3c2ca7920cf04fd7ce5a4b7..0f8354f26a4ba6b26c8abba88dbb5a499fcc4dc2 100644
--- a/base/metrics/stats_counters.h
+++ b/base/metrics/stats_counters.h
@@ -133,7 +133,7 @@ class BASE_EXPORT StatsCounterTimer : protected StatsCounter {
  public:
   // Constructs and starts the timer.
   explicit StatsCounterTimer(const std::string& name);
-  virtual ~StatsCounterTimer();
+  ~StatsCounterTimer() override;
 
   // Start the timer.
   void Start();
@@ -162,9 +162,9 @@ class BASE_EXPORT StatsRate : public StatsCounterTimer {
  public:
   // Constructs and starts the timer.
   explicit StatsRate(const std::string& name);
-  virtual ~StatsRate();
+  ~StatsRate() override;
 
-  virtual void Add(int value) override;
+  void Add(int value) override;
 
  private:
   StatsCounter counter_;