Standardize usage of virtual/override/final in extensions/

extensions/common/features/manifest_feature.h

Index: extensions/common/features/manifest_feature.h
diff --git a/extensions/common/features/manifest_feature.h b/extensions/common/features/manifest_feature.h
index b4d38085356e0b55a57419a0aff2dae688aaafbc..ab77d028df8f56dc24da3925b2434fd073c0a1ab 100644
--- a/extensions/common/features/manifest_feature.h
+++ b/extensions/common/features/manifest_feature.h
@@ -14,15 +14,15 @@ namespace extensions {
 class ManifestFeature : public SimpleFeature {
  public:
   ManifestFeature();
-  virtual ~ManifestFeature();
+  ~ManifestFeature() override;
 
-  virtual Feature::Availability IsAvailableToContext(
+  Feature::Availability IsAvailableToContext(
       const Extension* extension,
       Feature::Context context,
       const GURL& url,
       Feature::Platform platform) const override;
 
-  virtual std::string Parse(const base::DictionaryValue* value) override;
+  std::string Parse(const base::DictionaryValue* value) override;
 };
 
 }  // namespace extensions