Standardize usage of virtual/override/final specifiers in base/.

base/callback_unittest.cc

Index: base/callback_unittest.cc
diff --git a/base/callback_unittest.cc b/base/callback_unittest.cc
index e2a5d38a31cc4bd173087667bb3c075db1ff1270..5644b8515f0e538942d69c945cae2b56b0a075cd 100644
--- a/base/callback_unittest.cc
+++ b/base/callback_unittest.cc
@@ -66,8 +66,7 @@ class CallbackTest : public ::testing::Test {
         callback_b_(new FakeBindState2()) {
   }
 
-  virtual ~CallbackTest() {
-  }
+  ~CallbackTest() override {}
 
  protected:
   Callback<void(void)> callback_a_;