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

base/single_thread_task_runner.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_SINGLE_THREAD_TASK_RUNNER_H_
 #define BASE_SINGLE_THREAD_TASK_RUNNER_H_
 #pragma once
 
 #include "base/base_export.h"
 #include "base/sequenced_task_runner.h"
@@ skipping 14 matching lines @@
 //     and has a method Run() that runs each runnable task in FIFO
 //     order that must be run only from the thread the
 //     SingleThreadTaskRunner was created on.
 class BASE_EXPORT SingleThreadTaskRunner : public SequencedTaskRunner {
  public:
   // A more explicit alias to RunsTasksOnCurrentThread().
   bool BelongsToCurrentThread() const {
     return RunsTasksOnCurrentThread();
   }
 
+  // Gets the SingleThreadTaskRunner for the current thread.
+  static scoped_refptr<SingleThreadTaskRunner> current();
+
+  // Sets SingleThreadTaskRunner for the current thread. The caller
+  // must keep a reference to the task runner and call this method
+  // again with new_current=NULL before dropping that reference.
+  static void SetCurrent(SingleThreadTaskRunner* new_current);
+
  protected:
   virtual ~SingleThreadTaskRunner() {}
 };
 
 }  // namespace base
 
 #endif  // BASE_SERIAL_TASK_RUNNER_H_