Update {virtual,override,final} to follow C++11 style in ash.

ash/system/chromeos/tray_tracing.cc

Index: ash/system/chromeos/tray_tracing.cc
diff --git a/ash/system/chromeos/tray_tracing.cc b/ash/system/chromeos/tray_tracing.cc
index 279bea9f8637eb78e2253c806a2bd705bea7b172..b74bc5ae93fce20c8c7a65a7ba54d736832fb48a 100644
--- a/ash/system/chromeos/tray_tracing.cc
+++ b/ash/system/chromeos/tray_tracing.cc
@@ -43,11 +43,11 @@ class DefaultTracingView : public ActionableView {
     AddChildView(label_);
   }
 
-  virtual ~DefaultTracingView() {}
+  ~DefaultTracingView() override {}
 
  private:
   // Overridden from ActionableView.
-  virtual bool PerformAction(const ui::Event& event) override {
+  bool PerformAction(const ui::Event& event) override {
     Shell::GetInstance()->system_tray_delegate()->ShowChromeSlow();
     return true;
   }