Standardize usage of virtual/override/final in base/

base/files/important_file_writer_unittest.cc

Index: base/files/important_file_writer_unittest.cc
diff --git a/base/files/important_file_writer_unittest.cc b/base/files/important_file_writer_unittest.cc
index 71242eeaabe59d5ded63d6696bde84371f848364..96d0d059bb80d95c59ae57fcf4e6429173a17f0c 100644
--- a/base/files/important_file_writer_unittest.cc
+++ b/base/files/important_file_writer_unittest.cc
@@ -33,7 +33,7 @@ class DataSerializer : public ImportantFileWriter::DataSerializer {
   explicit DataSerializer(const std::string& data) : data_(data) {
   }
 
-  virtual bool SerializeData(std::string* output) override {
+  bool SerializeData(std::string* output) override {
     output->assign(data_);
     return true;
   }