Add Support to Get a COM STA Task Runner from the Task Scheduler API

base/test/scoped_task_scheduler.h

 // Copyright 2016 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_TEST_SCOPED_TASK_SCHEDULER_H_
 #define BASE_TEST_SCOPED_TASK_SCHEDULER_H_
 
 #include "base/macros.h"
 #include "base/threading/thread_checker.h"
 
@@ skipping 39 matching lines @@
 // runs "D" because it's BLOCK_SHUTDOWN. "C" is skipped.
 class ScopedTaskScheduler {
  public:
   // Registers a synchronous TaskScheduler on a thread that doesn't have a
   // MessageLoop.
   //
   // This constructor handles most common cases.
   ScopedTaskScheduler();
 
   // Registers a synchronous TaskScheduler on a thread that already has a
-  // |message_loop|. Calling RunLoop::Run/RunUntilIdle() on the thread where
-  // this lives runs the MessageLoop and TaskScheduler tasks in posting order.
+  // |message_loop| assumed to be associated with the caller's thread. Calling
+  // RunLoop::Run/RunUntilIdle() on the thread where this lives runs the
+  // MessageLoop and TaskScheduler tasks in posting order.
   //
   // In general, you don't need a ScopedTaskScheduler and a MessageLoop because
   // ScopedTaskScheduler provides most MessageLoop features.
   //
   //     ScopedTaskScheduler scoped_task_scheduler;
   //     ThreadTaskRunnerHandle::Get()->PostTask(FROM_HERE, Bind(&Task));
   //     RunLoop().RunUntilIdle();  // Runs Task.
   //
   //     is equivalent to
   //
@@ skipping 21 matching lines @@
   const TaskScheduler* task_scheduler_ = nullptr;
   ThreadChecker thread_checker_;
 
   DISALLOW_COPY_AND_ASSIGN(ScopedTaskScheduler);
 };
 
 }  // namespace test
 }  // namespace base
 
 #endif  // BASE_TEST_SCOPED_TASK_SCHEDULER_H_