Standardize usage of virtual/override/final in base/

base/single_thread_task_runner.h

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef BASE_SINGLE_THREAD_TASK_RUNNER_H_
 #define BASE_SINGLE_THREAD_TASK_RUNNER_H_
 
 #include "base/base_export.h"
 #include "base/sequenced_task_runner.h"
 
@@ skipping 11 matching lines @@
 //     be processed. This allows TaskRunner-oriented code run on threads
 //     running other kinds of message loop, e.g. Jingle threads.
 class BASE_EXPORT SingleThreadTaskRunner : public SequencedTaskRunner {
  public:
   // A more explicit alias to RunsTasksOnCurrentThread().
   bool BelongsToCurrentThread() const {
     return RunsTasksOnCurrentThread();
   }
 
  protected:
-  virtual ~SingleThreadTaskRunner() {}
+  ~SingleThreadTaskRunner() override {}
 };
 
 }  // namespace base
 
 #endif  // BASE_SINGLE_THREAD_TASK_RUNNER_H_