Introduce SingleThreadTaskRunnerThreadMode

base/task_scheduler/single_thread_task_runner_thread_mode.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_TASK_SCHEDULER_SINGLE_THREAD_TASK_RUNNER_THREAD_MODE_H_
+#define BASE_TASK_SCHEDULER_SINGLE_THREAD_TASK_RUNNER_THREAD_MODE_H_
+
+namespace base {
+
+enum class SingleThreadTaskRunnerThreadMode {
+  // Allow the SingleThreadTaskRunner's thread to be shared with others,
+  // allowing for efficient use of thread resources when this
+  // SingleThreadTaskRunner is idle. This is the default mode and is
+  // recommended for most code.
+  SHARED,
+  // Dedicate a single thread for this SingleThreadTaskRunner. No other tasks
+  // from any other source will be scheduled on the thread backing
+  // the SingleThreadTaskRunner. Use sparingly as this reserves an entire
+  // thread for this SingleThreadTaskRunner.
+  DEDICATED,
+};
+
+}  // namespace base
+
+#endif  // BASE_TASK_SCHEDULER_SINGLE_THREAD_TASK_RUNNER_THREAD_MODE_H_