Make MessageLoopProxy::current() usable on threads that don't have MessageLoop.

base/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_PROXY_H_
 #define BASE_MESSAGE_LOOP_PROXY_H_
 #pragma once
 
 #include "base/base_export.h"
 #include "base/compiler_specific.h"
 #include "base/memory/ref_counted.h"
-#include "base/single_thread_task_runner.h"
+#include "base/thread_main_task_runner.h"
 
 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 {
+class BASE_EXPORT MessageLoopProxy : public ThreadMainTaskRunner {
  public:
   // Gets the MessageLoopProxy for the current message loop, creating one if
   // needed.
   static scoped_refptr<MessageLoopProxy> current();
 
  protected:
   MessageLoopProxy();
   virtual ~MessageLoopProxy();
 };
 
 }  // namespace base
 
 #endif  // BASE_MESSAGE_LOOP_PROXY_H_