Standardize usage of virtual/override/final in extensions/

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/664933004

Cr-Commit-Position: refs/heads/master@{#300475}
diff --git a/extensions/renderer/runtime_custom_bindings.h b/extensions/renderer/runtime_custom_bindings.h
index 70f4d03..17e96bb 100644
--- a/extensions/renderer/runtime_custom_bindings.h
+++ b/extensions/renderer/runtime_custom_bindings.h
@@ -16,7 +16,7 @@
  public:
   explicit RuntimeCustomBindings(ScriptContext* context);
 
-  virtual ~RuntimeCustomBindings();
+  ~RuntimeCustomBindings() override;
 
   // Creates a new messaging channel to the given extension.
   void OpenChannelToExtension(const v8::FunctionCallbackInfo<v8::Value>& args);