Standardize usage of virtual/override/final in base/

BUG=417463
TBR=danakj@chromium.org

Review URL: https://codereview.chromium.org/668783004

Cr-Commit-Position: refs/heads/master@{#300447}


CrOS-Libchrome-Original-Commit: 5648818ef9d5ef5fda79cd5dde2e42567af07785
diff --git a/base/sequenced_task_runner.h b/base/sequenced_task_runner.h
index f6ca646..71dcf05 100644
--- a/base/sequenced_task_runner.h
+++ b/base/sequenced_task_runner.h
@@ -139,7 +139,7 @@
   }
 
  protected:
-  virtual ~SequencedTaskRunner() {}
+  ~SequencedTaskRunner() override {}
 
  private:
   template <class T, class R> friend class subtle::DeleteHelperInternal;