Make new TaskRunner, SequencedTaskRunner, and SingleThreadTaskRunner interfaces

base/single_thread_executor.h

Index: base/single_thread_executor.h
diff --git a/base/single_thread_executor.h b/base/single_thread_executor.h
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+// 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_EXECUTOR_H_
+#define BASE_SINGLE_THREAD_EXECUTOR_H_
+#pragma once
+
+#include "base/base_export.h"
+#include "base/serial_executor.h"
+
+namespace base {
+
+// A SingleThreadExecutor is a SerialExecutor with one more guarantee;
+// namely, that all tasks are executed on a single specific thread.
+//
+// Some theoretical implementations of SingleThreadExecutor:
+//
+//   - A SingleThreadExecutor that uses a single worker thread to run
+//     submitted tasks (i.e., a message loop).
+//
+//   - A SingleThreadExecutor that stores the list of submitted tasks
+//     and has a method Run() that executes each runnable task in FIFO
+//     order that must be run only from the thread the
+//     SingleThreadExecutor was created on.
+class BASE_EXPORT SingleThreadExecutor : public SerialExecutor {
+public:
+  // A more explicit alias to MayRunTasksOnCurrentThread().
+  bool BelongsToCurrentThread() const {
+    return MayRunTasksOnCurrentThread();
+  }
+};
+
+}  // namespace base
+
+#endif  // BASE_SERIAL_EXECUTOR_H_