Standardize usage of virtual/override/final in chrome/browser/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/666153002

Cr-Commit-Position: refs/heads/master@{#300474}
diff --git a/chrome/browser/extensions/extension_web_ui.h b/chrome/browser/extensions/extension_web_ui.h
index cecd6b3a..0d44b0e 100644
--- a/chrome/browser/extensions/extension_web_ui.h
+++ b/chrome/browser/extensions/extension_web_ui.h
@@ -37,7 +37,7 @@
 
   ExtensionWebUI(content::WebUI* web_ui, const GURL& url);
 
-  virtual ~ExtensionWebUI();
+  ~ExtensionWebUI() override;
 
   virtual extensions::BookmarkManagerPrivateDragEventRouter*
       bookmark_manager_private_drag_event_router();