Update {virtual,override} to follow C++11 style in chrome.

chrome/renderer/prefetch_helper.h

 // Copyright (c) 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 CHROME_RENDERER_PREFETCH_HELPER_H_
 #define CHROME_RENDERER_PREFETCH_HELPER_H_
 
 #include <set>
 
 #include "content/public/renderer/render_frame_observer.h"
 #include "third_party/WebKit/public/platform/WebURLLoader.h"
 #include "third_party/WebKit/public/platform/WebURLLoaderClient.h"
 #include "url/gurl.h"
 
 namespace prefetch {
 
 // Helper class initiating prefetches on behalf of a RenderFrame.
 class PrefetchHelper : public content::RenderFrameObserver,
                        public blink::WebURLLoaderClient {
  public:
   explicit PrefetchHelper(content::RenderFrame* render_frame);
-  virtual ~PrefetchHelper();
+  ~PrefetchHelper() override;
 
   // blink::WebURLLoaderClient implementation
-  virtual void didFinishLoading(blink::WebURLLoader* loader,
-                                double finishTime,
-                                int64_t totalEncodedDataLength) override;
-  virtual void didFail(blink::WebURLLoader* loader,
-                       const blink::WebURLError& error) override;
+  void didFinishLoading(blink::WebURLLoader* loader,
+                        double finishTime,
+                        int64_t totalEncodedDataLength) override;
+  void didFail(blink::WebURLLoader* loader,
+               const blink::WebURLError& error) override;
 
  private:
   // RenderViewObserver implementation
   bool OnMessageReceived(const IPC::Message& message) override;
 
   void OnPrefetch(const GURL& url);
 
   std::set<blink::WebURLLoader*> loader_set_;
 
   DISALLOW_COPY_AND_ASSIGN(PrefetchHelper);
 };
 
 }  // namespace prefetch
 
 #endif  // CHROME_RENDERER_PREFETCH_HELPER_H_