Standardize usage of virtual/override/final in base/

base/debug/trace_event_system_stats_monitor.cc

Index: base/debug/trace_event_system_stats_monitor.cc
diff --git a/base/debug/trace_event_system_stats_monitor.cc b/base/debug/trace_event_system_stats_monitor.cc
index b2bf2ae8fd6012d68f3a3016fc25995c360fff67..9cbefd8d6cc14af406ce08d8ed33d3b20ac904f9 100644
--- a/base/debug/trace_event_system_stats_monitor.cc
+++ b/base/debug/trace_event_system_stats_monitor.cc
@@ -31,12 +31,12 @@ class SystemStatsHolder : public base::debug::ConvertableToTraceFormat {
   void GetSystemProfilingStats();
 
   // base::debug::ConvertableToTraceFormat overrides:
-  virtual void AppendAsTraceFormat(std::string* out) const override {
+  void AppendAsTraceFormat(std::string* out) const override {
     AppendSystemProfileAsTraceFormat(system_stats_, out);
   }
 
  private:
-  virtual ~SystemStatsHolder() { }
+  ~SystemStatsHolder() override {}
 
   SystemMetrics system_stats_;