Adding a comment for MessageLoopProxy deprecation in message_loop_proxy.h

base/message_loop/message_loop_proxy.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_LOOP_MESSAGE_LOOP_PROXY_H_
 #define BASE_MESSAGE_LOOP_MESSAGE_LOOP_PROXY_H_
 
 #include "base/base_export.h"
 #include "base/compiler_specific.h"
 #include "base/memory/ref_counted.h"
 #include "base/single_thread_task_runner.h"
 
+// MessageLoopProxy is deprecated. Code should prefer to depend on TaskRunner
+// (or the various specializations) for passing task runners around, and should
+// use ThreadTaskRunnerHandle::Get() to get the thread's associated task runner.
+//
+// See http://crbug.com/391045 for more details.
+// Example for these changes:
+//
+// base::MessageLoopProxy::current() -> base::ThreadTaskRunnerHandle::Get()
+// scoped_refptr<base::MessageLoopProxy> ->
+//     scoped_refptr<base::SingleThreadTaskRunner>
+// base::MessageLoopProxy -> base::SingleThreadTaskRunner
+
 namespace base {
 
 // This class provides a thread-safe refcounted interface to the Post* methods
 // of a message loop. This class can outlive the target message loop.
 // MessageLoopProxy objects are constructed automatically for all MessageLoops.
 // So, to access them, you can use any of the following:
 //   Thread::message_loop_proxy()
 //   MessageLoop::current()->message_loop_proxy()
 //   MessageLoopProxy::current()
 //
 // TODO(akalin): Now that we have the *TaskRunner interfaces, we can
 // merge this with MessageLoopProxyImpl.
 class BASE_EXPORT MessageLoopProxy : public SingleThreadTaskRunner {
  public:
   // Gets the MessageLoopProxy for the current message loop, creating one if
   // needed.
   static scoped_refptr<MessageLoopProxy> current();
 
  protected:
   MessageLoopProxy();
   ~MessageLoopProxy() override;
 };
 
 }  // namespace base
 
 #endif  // BASE_MESSAGE_LOOP_MESSAGE_LOOP_PROXY_H_