Standardize usage of virtual/override/final in extensions/

extensions/renderer/extension_helper.h

Index: extensions/renderer/extension_helper.h
diff --git a/extensions/renderer/extension_helper.h b/extensions/renderer/extension_helper.h
index c61bd7c5231f6a130c5627ffd88408f941cc5cae..7e660757d6d1c0eeab281efe3bef3b254d5333cb 100644
--- a/extensions/renderer/extension_helper.h
+++ b/extensions/renderer/extension_helper.h
@@ -43,7 +43,7 @@ class ExtensionHelper
       const std::string& extension_id);
 
   ExtensionHelper(content::RenderView* render_view, Dispatcher* dispatcher);
-  virtual ~ExtensionHelper();
+  ~ExtensionHelper() override;
 
   int tab_id() const { return tab_id_; }
   int browser_window_id() const { return browser_window_id_; }
@@ -52,14 +52,14 @@ class ExtensionHelper
 
  private:
   // RenderViewObserver implementation.
-  virtual bool OnMessageReceived(const IPC::Message& message) override;
-  virtual void DidCreateDocumentElement(blink::WebLocalFrame* frame) override;
-  virtual void DidMatchCSS(
+  bool OnMessageReceived(const IPC::Message& message) override;
+  void DidCreateDocumentElement(blink::WebLocalFrame* frame) override;
+  void DidMatchCSS(
       blink::WebLocalFrame* frame,
       const blink::WebVector<blink::WebString>& newly_matching_selectors,
       const blink::WebVector<blink::WebString>& stopped_matching_selectors)
       override;
-  virtual void DraggableRegionsChanged(blink::WebFrame* frame) override;
+  void DraggableRegionsChanged(blink::WebFrame* frame) override;
 
   void OnExtensionResponse(int request_id, bool success,
                            const base::ListValue& response,