Standardize usage of virtual/override/final specifiers.

ash/system/tray/default_system_tray_delegate.cc

Index: ash/system/tray/default_system_tray_delegate.cc
diff --git a/ash/system/tray/default_system_tray_delegate.cc b/ash/system/tray/default_system_tray_delegate.cc
index bd4bb374a9b54d92a244b2cd21af6ce071db6bed..6542d382e2fe78d44e0f88c9413ce795702798a5 100644
--- a/ash/system/tray/default_system_tray_delegate.cc
+++ b/ash/system/tray/default_system_tray_delegate.cc
@@ -19,15 +19,15 @@ namespace {
 class DefaultVolumnControlDelegate : public VolumeControlDelegate {
  public:
   DefaultVolumnControlDelegate() {}
-  virtual ~DefaultVolumnControlDelegate() {}
+  ~DefaultVolumnControlDelegate() override {}
 
-  virtual bool HandleVolumeMute(const ui::Accelerator& accelerator) override {
+  bool HandleVolumeMute(const ui::Accelerator& accelerator) override {
     return true;
   }
-  virtual bool HandleVolumeDown(const ui::Accelerator& accelerator) override {
+  bool HandleVolumeDown(const ui::Accelerator& accelerator) override {
     return true;
   }
-  virtual bool HandleVolumeUp(const ui::Accelerator& accelerator) override {
+  bool HandleVolumeUp(const ui::Accelerator& accelerator) override {
     return true;
   }