Fixed the racy code around the message-only window in base::MessagePumpForUI on Windows.

base/message_pump.h

 // Copyright (c) 2012 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_PUMP_H_
 #define BASE_MESSAGE_PUMP_H_
 
 #include "base/base_export.h"
 #include "base/memory/ref_counted.h"
 
@@ skipping 101 matching lines @@
   // DoWork callback is already scheduled.  This method may be called from any
   // thread.  Once this call is made, DoWork should not be "starved" at least
   // until it returns a value of false.
   virtual void ScheduleWork() = 0;
 
   // Schedule a DoDelayedWork callback to happen at the specified time,
   // cancelling any pending DoDelayedWork callback.  This method may only be
   // used on the thread that called Run.
   virtual void ScheduleDelayedWork(const TimeTicks& delayed_work_time) = 0;
 
+  // Called directly by MessageLoop::~MessageLoop to clean up resources that
+  // must be freed on the MessageLoop's thread.
+  virtual void WillDestroyCurrentMessageLoop() {}
+
  protected:
   virtual ~MessagePump();
   friend class RefCountedThreadSafe<MessagePump>;
 };
 
 }  // namespace base
 
 #endif  // BASE_MESSAGE_PUMP_H_