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

base/memory/singleton_unittest.cc

Index: base/memory/singleton_unittest.cc
diff --git a/base/memory/singleton_unittest.cc b/base/memory/singleton_unittest.cc
index 0e9ecdc03aaf3e23919de9febe8468194ef0d4a1..dbff007ada7ecc27045acdff7082fb448024dab1 100644
--- a/base/memory/singleton_unittest.cc
+++ b/base/memory/singleton_unittest.cc
@@ -153,7 +153,7 @@ class SingletonTest : public testing::Test {
  public:
   SingletonTest() {}
 
-  virtual void SetUp() override {
+  void SetUp() override {
     non_leak_called_ = false;
     leaky_called_ = false;
     static_called_ = false;