Make new TaskRunner, SequencedTaskRunner, and SingleThreadTaskRunner interfaces

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
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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_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.
+//
+// Some theoretical implementations of SingleThreadTaskRunner:
+//
+//   - A SingleThreadTaskRunner that uses a single worker thread to
+//     run posted tasks (i.e., a message loop).
+//
+//   - A SingleThreadTaskRunner that stores the list of posted tasks
+//     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();
+  }
+};
+
+}  // namespace base
+
+#endif  // BASE_SERIAL_TASK_RUNNER_H_