Standardize usage of virtual/override/final specifiers.

ash/wm/screen_dimmer_unittest.cc

Index: ash/wm/screen_dimmer_unittest.cc
diff --git a/ash/wm/screen_dimmer_unittest.cc b/ash/wm/screen_dimmer_unittest.cc
index 91ecf4e8869fac30ff9c0cd22a1ccf58cab9300b..244beecc7057d408ac13ed482538c0c7c41423f8 100644
--- a/ash/wm/screen_dimmer_unittest.cc
+++ b/ash/wm/screen_dimmer_unittest.cc
@@ -18,9 +18,9 @@ namespace test {
 class ScreenDimmerTest : public AshTestBase {
  public:
   ScreenDimmerTest() : dimmer_(NULL) {}
-  virtual ~ScreenDimmerTest() {}
+  ~ScreenDimmerTest() override {}
 
-  virtual void SetUp() override {
+  void SetUp() override {
     AshTestBase::SetUp();
     dimmer_ = Shell::GetPrimaryRootWindowController()->screen_dimmer();
     test_api_.reset(new ScreenDimmer::TestApi(dimmer_));