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

base/single_thread_task_runner.h

Index: base/single_thread_task_runner.h
diff --git a/base/single_thread_task_runner.h b/base/single_thread_task_runner.h
index c87e5f7ff1438c4599b18d5c146b2de89c9476ef..46d2ab418310a7f52b57b1bc87dc647a04df3eeb 100644
--- a/base/single_thread_task_runner.h
+++ b/base/single_thread_task_runner.h
@@ -32,6 +32,14 @@ class BASE_EXPORT SingleThreadTaskRunner : public SequencedTaskRunner {
     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() {}
 };