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

chrome/renderer/prefetch_helper.h

Index: chrome/renderer/prefetch_helper.h
diff --git a/chrome/renderer/prefetch_helper.h b/chrome/renderer/prefetch_helper.h
index ec44b7a7c0b6b9d77b95a579745cb8358cb45ad9..f19cf6704941907ae3c8e282a2c0fa7ac0345e25 100644
--- a/chrome/renderer/prefetch_helper.h
+++ b/chrome/renderer/prefetch_helper.h
@@ -19,14 +19,14 @@ 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