Introduce TaskRunnerHandle.

base/threading/task_runner_handle_internal.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_THREADING_TASK_RUNNER_HANDLE_INTERNAL_H_
+#define BASE_THREADING_TASK_RUNNER_HANDLE_INTERNAL_H_
+
+#include "base/macros.h"
+#include "base/memory/ref_counted.h"
+
+namespace base {
+
+class SequencedTaskRunner;
+class SingleThreadTaskRunner;
+class TaskRunner;
+
+namespace internal {
+
+// Holds the state associated with one of the TaskScope variants. Stores the
+// relevant pointer in a generic TaskRunner pointer and remembers the type
+// through an enum which makes for fast lookup of which levels of TaskRunners
+// are available.
+struct TaskScopeState {
+  TaskScopeState(scoped_refptr<TaskRunner> task_runner);
+  TaskScopeState(scoped_refptr<SequencedTaskRunner> sequenced_task_runner);
+  TaskScopeState(
+      scoped_refptr<SingleThreadTaskRunner> single_thread_task_runner);
+
+  ~TaskScopeState();
+
+  // TaskScope types sorted from least restrictive to most restrictive. Order
+  // is used for quick lookup of available TaskScope type on current thread.
+  enum Type { PARALLEL, SEQUENCED, SINGLE_THREADED };
+
+  const Type type;
+
+  // A generic TaskRunner to store the potentially more specific TaskRunner
+  // which can safely be upcasted if |type| allows.
+  const scoped_refptr<TaskRunner> task_runner;
+
+ private:
+  DISALLOW_COPY_AND_ASSIGN(TaskScopeState);
+};
+
+}  // namespace internal
+}  // namespace base
+
+#endif  // BASE_THREADING_TASK_RUNNER_HANDLE_INTERNAL_H_