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

content/browser/system_message_window_win_unittest.cc

Index: content/browser/system_message_window_win_unittest.cc
diff --git a/content/browser/system_message_window_win_unittest.cc b/content/browser/system_message_window_win_unittest.cc
index f317f7be7cc9e9aa5425126666f454bf4362e041..fac8f2baafd74198000751a5a2cf3c8fc3305449 100644
--- a/content/browser/system_message_window_win_unittest.cc
+++ b/content/browser/system_message_window_win_unittest.cc
@@ -18,10 +18,10 @@ namespace content {
 
 class SystemMessageWindowWinTest : public testing::Test {
  public:
-  virtual ~SystemMessageWindowWinTest() { }
+  ~SystemMessageWindowWinTest() override {}
 
  protected:
-  virtual void SetUp() override {
+  void SetUp() override {
     system_monitor_.AddDevicesChangedObserver(&observer_);
   }