Remove #pragma once. Just from base/ for now.

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"
 
 namespace base {
 
 // A SingleThreadTaskRunner is a SequencedTaskRunner with one more
 // guarantee; namely, that all tasks are run on a single dedicated
 // thread.  Most use cases require only a SequencedTaskRunner, unless
 // there is a specific need to run tasks on only a single dedicated.
@@ skipping 14 matching lines @@
     return RunsTasksOnCurrentThread();
   }
 
  protected:
   virtual ~SingleThreadTaskRunner() {}
 };
 
 }  // namespace base
 
 #endif  // BASE_SERIAL_TASK_RUNNER_H_