Use exactly one of final, override, and virtual

util/file/string_file_writer.h

Index: util/file/string_file_writer.h
diff --git a/util/file/string_file_writer.h b/util/file/string_file_writer.h
index 4b4002bd13e8473d742c9880e89619ef0d941a93..fa1e7de938cac21dac319189515f7f47d6e7393d 100644
--- a/util/file/string_file_writer.h
+++ b/util/file/string_file_writer.h
@@ -45,9 +45,9 @@ class StringFileWriter : public FileWriterInterface {
   void Reset();
 
   // FileWriterInterface:
-  virtual bool Write(const void* data, size_t size) override;
-  virtual bool WriteIoVec(std::vector<WritableIoVec>* iovecs) override;
-  virtual off_t Seek(off_t offset, int whence) override;
+  bool Write(const void* data, size_t size) override;
+  bool WriteIoVec(std::vector<WritableIoVec>* iovecs) override;
+  off_t Seek(off_t offset, int whence) override;
 
  private:
   //! \brief The virtual file’s contents.