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

base/message_loop_proxy_impl.h

 // Copyright (c) 2010 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_MESSAGE_LOOP_PROXY_IMPL_H_
 #define BASE_MESSAGE_LOOP_PROXY_IMPL_H_
 #pragma once
 
 #include "base/lock.h"
 #include "base/message_loop.h"
@@ skipping 15 matching lines @@
   virtual bool PostDelayedTask(const tracked_objects::Location& from_here,
                                Task* task, int64 delay_ms);
   virtual bool PostNonNestableTask(const tracked_objects::Location& from_here,
                                    Task* task);
   virtual bool PostNonNestableDelayedTask(
       const tracked_objects::Location& from_here,
       Task* task,
       int64 delay_ms);
   virtual bool BelongsToCurrentThread();
 
-// MessageLoop::DestructionObserver implementation
-  void WillDestroyCurrentMessageLoop();
+  // MessageLoop::DestructionObserver implementation
+  virtual void WillDestroyCurrentMessageLoop();
 
  protected:
   // Override OnDestruct so that we can delete the object on the target message
   // loop if it still exists.
   virtual void OnDestruct() const;
 
  private:
   MessageLoopProxyImpl();
   bool PostTaskHelper(const tracked_objects::Location& from_here,
                       Task* task, int64 delay_ms, bool nestable);
 
   // For the factory method to work
   friend class MessageLoopProxy;
 
   // The lock that protects access to target_message_loop_.
   mutable Lock message_loop_lock_;
   MessageLoop* target_message_loop_;
 
   DISALLOW_COPY_AND_ASSIGN(MessageLoopProxyImpl);
 };
 
 }  // namespace base
 
 #endif  // BASE_MESSAGE_LOOP_PROXY_IMPL_H_