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

content/public/test/mock_download_item.h

Index: content/public/test/mock_download_item.h
diff --git a/content/public/test/mock_download_item.h b/content/public/test/mock_download_item.h
index 19ebe62b5f8411be8b63626c87621f98e775a394..2b1f8b7cca44bd274d42e160323c2b38ff202158 100644
--- a/content/public/test/mock_download_item.h
+++ b/content/public/test/mock_download_item.h
@@ -19,13 +19,13 @@ namespace content {
 class MockDownloadItem : public DownloadItem {
  public:
   MockDownloadItem();
-  virtual ~MockDownloadItem();
+  ~MockDownloadItem() override;
 
   // Management of observer lists is common in tests. So Add/RemoveObserver
   // methods are not mocks. In addition, any registered observers will receive a
   // OnDownloadDestroyed() notification when the mock is destroyed.
-  virtual void AddObserver(Observer* observer) override;
-  virtual void RemoveObserver(Observer* observer) override;
+  void AddObserver(Observer* observer) override;
+  void RemoveObserver(Observer* observer) override;
 
   // Dispatches an OnDownloadOpened() notification to observers.
   void NotifyObserversDownloadOpened();