Standardize usage of virtual/override/final in extensions/

extensions/browser/extension_system_provider.h

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef EXTENSIONS_BROWSER_EXTENSION_SYSTEM_PROVIDER_H_
 #define EXTENSIONS_BROWSER_EXTENSION_SYSTEM_PROVIDER_H_
 
 #include "components/keyed_service/content/browser_context_keyed_service_factory.h"
 
 class BrowserContextDependencyManager;
 
 namespace content {
 class BrowserContext;
 }
 
 namespace extensions {
 class ExtensionSystem;
 
 // An ExtensionSystemProvider maps a BrowserContext to its ExtensionSystem.
 // Different applications may use this to provide differing implementations
 // of ExtensionSystem.
 // TODO(yoz): Rename to ExtensionSystemFactory.
 class ExtensionSystemProvider : public BrowserContextKeyedServiceFactory {
  public:
   ExtensionSystemProvider(const char* name,
                           BrowserContextDependencyManager* manager);
-  virtual ~ExtensionSystemProvider();
+  ~ExtensionSystemProvider() override;
 
   virtual ExtensionSystem* GetForBrowserContext(
       content::BrowserContext* context) = 0;
 };
 
 }  // namespace extensions
 
 #endif  // EXTENSIONS_BROWSER_EXTENSION_SYSTEM_PROVIDER_H_