Standardize usage of virtual/override/final in net/

This patch was automatically generated by applying clang fixit hints
generated by the plugin to the source tree.

BUG=417463
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Review URL: https://codereview.chromium.org/667923003

Cr-Commit-Position: refs/heads/master@{#300466}
diff --git a/net/base/network_config_watcher_mac.cc b/net/base/network_config_watcher_mac.cc
index 7d2377c..bdf75c3 100644
--- a/net/base/network_config_watcher_mac.cc
+++ b/net/base/network_config_watcher_mac.cc
@@ -31,12 +31,12 @@
 class NetworkConfigWatcherMacThread : public base::Thread {
  public:
   NetworkConfigWatcherMacThread(NetworkConfigWatcherMac::Delegate* delegate);
-  virtual ~NetworkConfigWatcherMacThread();
+  ~NetworkConfigWatcherMacThread() override;
 
  protected:
   // base::Thread
-  virtual void Init() override;
-  virtual void CleanUp() override;
+  void Init() override;
+  void CleanUp() override;
 
  private:
   // The SystemConfiguration calls in this function can lead to contention early