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

chrome/browser/extensions/extension_message_service.h

Index: chrome/browser/extensions/extension_message_service.h
diff --git a/chrome/browser/extensions/extension_message_service.h b/chrome/browser/extensions/extension_message_service.h
index 9135a1cb2cbfe6024d54c8cc8be1f989f58cd56a..30c771d0568d660b2fb265e76c17353c1b6779ee 100644
--- a/chrome/browser/extensions/extension_message_service.h
+++ b/chrome/browser/extensions/extension_message_service.h
@@ -134,9 +134,9 @@ class ExtensionMessageService
                         bool notify_other_port);
 
   // NotificationObserver interface.
-  void Observe(NotificationType type,
-               const NotificationSource& source,
-               const NotificationDetails& details);
+  virtual void Observe(NotificationType type,
+                       const NotificationSource& source,
+                       const NotificationDetails& details);
 
   // An IPC sender that might be in our list of channels has closed.
   void OnSenderClosed(IPC::Message::Sender* sender);