Add the "virtual" keyword on method overrides that are missing it.

chrome/browser/worker_host/worker_service.h

Index: chrome/browser/worker_host/worker_service.h
diff --git a/chrome/browser/worker_host/worker_service.h b/chrome/browser/worker_host/worker_service.h
index 744f1166355e59115a5e49bfa54057ad9603ba0a..994d11f477d37105f2fd2831c937ab8bf3a6f5e1 100644
--- a/chrome/browser/worker_host/worker_service.h
+++ b/chrome/browser/worker_host/worker_service.h
@@ -142,9 +142,9 @@ class WorkerService : public NotificationObserver {
       int renderer_id, int render_view_route_id, bool* hit_total_worker_limit);
 
   // NotificationObserver interface.
-  void Observe(NotificationType type,
-               const NotificationSource& source,
-               const NotificationDetails& details);
+  virtual void Observe(NotificationType type,
+                       const NotificationSource& source,
+                       const NotificationDetails& details);
 
   // Notifies us that a process that's talking to a worker has shut down.
   void SenderShutdown(IPC::Message::Sender* sender);