Mac Chromium style checker cleanup

chrome/browser/ui/cocoa/download/download_started_animation_mac.mm

Index: chrome/browser/ui/cocoa/download/download_started_animation_mac.mm
diff --git a/chrome/browser/ui/cocoa/download/download_started_animation_mac.mm b/chrome/browser/ui/cocoa/download/download_started_animation_mac.mm
index 14305d11a5cc5da4df0ff62d5c067cb0d4d699d5..901a755942b2bca5a22766b974957007b62b09c5 100644
--- a/chrome/browser/ui/cocoa/download/download_started_animation_mac.mm
+++ b/chrome/browser/ui/cocoa/download/download_started_animation_mac.mm
@@ -65,9 +65,10 @@ class DownloadAnimationWebObserver : public content::NotificationObserver {
 
   // Runs when a tab is hidden or destroyed. Let our owner know we should end
   // the animation.
-  void Observe(int type,
-               const content::NotificationSource& source,
-               const content::NotificationDetails& details) {
+  virtual void Observe(
+      int type,
+      const content::NotificationSource& source,
+      const content::NotificationDetails& details) OVERRIDE {
     if (type == content::NOTIFICATION_WEB_CONTENTS_VISIBILITY_CHANGED) {
       bool visible = *content::Details<bool>(details).ptr();
       if (visible)