Standardize usage of virtual/override/final specifiers.

ash/system/tray/tray_empty.cc

Index: ash/system/tray/tray_empty.cc
diff --git a/ash/system/tray/tray_empty.cc b/ash/system/tray/tray_empty.cc
index 04ca207a9fafb9140de03815c15ca36628e31f2b..821632233af35a6562ddaa3aabad9b63d40bb2da 100644
--- a/ash/system/tray/tray_empty.cc
+++ b/ash/system/tray/tray_empty.cc
@@ -14,11 +14,10 @@ namespace {
 class EmptyBackground : public views::Background {
  public:
   EmptyBackground() {}
-  virtual ~EmptyBackground() {}
+  ~EmptyBackground() override {}
 
  private:
-  virtual void Paint(gfx::Canvas* canvas, views::View* view) const override {
-  }
+  void Paint(gfx::Canvas* canvas, views::View* view) const override {}
 
   DISALLOW_COPY_AND_ASSIGN(EmptyBackground);
 };