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

base/waitable_event_watcher.h

 // Copyright (c) 2006-2008 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef BASE_WAITABLE_EVENT_WATCHER_H_
 #define BASE_WAITABLE_EVENT_WATCHER_H_
 #pragma once
 
 #include "build/build_config.h"
 
@@ skipping 127 matching lines @@
   };
 
   void OnObjectSignaled();
 
   ObjectWatcherHelper helper_;
   ObjectWatcher watcher_;
 #else
   // ---------------------------------------------------------------------------
   // Implementation of MessageLoop::DestructionObserver
   // ---------------------------------------------------------------------------
-  void WillDestroyCurrentMessageLoop();
+  virtual void WillDestroyCurrentMessageLoop();
 
   MessageLoop* message_loop_;
   scoped_refptr<Flag> cancel_flag_;
   AsyncWaiter* waiter_;
   AsyncCallbackTask* callback_task_;
   scoped_refptr<WaitableEvent::WaitableEventKernel> kernel_;
 #endif
 
   Delegate* delegate_;
 };
 
 }  // namespace base
 
 #endif  // BASE_WAITABLE_EVENT_WATCHER_H_