Standardize usage of virtual/override/final in base/

BUG=417463
[email protected]

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

Cr-Commit-Position: refs/heads/master@{#300447}
diff --git a/base/task_runner.cc b/base/task_runner.cc
index f2c64f3..262e1f8 100644
--- a/base/task_runner.cc
+++ b/base/task_runner.cc
@@ -20,8 +20,8 @@
   explicit PostTaskAndReplyTaskRunner(TaskRunner* destination);
 
  private:
-  virtual bool PostTask(const tracked_objects::Location& from_here,
-                        const Closure& task) override;
+  bool PostTask(const tracked_objects::Location& from_here,
+                const Closure& task) override;
 
   // Non-owning.
   TaskRunner* destination_;