Add base::test::ScopedAsyncTaskScheduler.

base/test/scoped_async_task_scheduler.h

+// Copyright 2017 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_TEST_SCOPED_ASYNC_TASK_SCHEDULER_H_
+#define BASE_TEST_SCOPED_ASYNC_TASK_SCHEDULER_H_
+
+#include "base/macros.h"
+
+namespace base {
+
+class TaskScheduler;
+
+namespace test {
+
+// Allows usage of the base/task_scheduler/post_task.h API within its scope.
+// Tasks run asynchronously, one at a time.
+//
+// To wait until all posted tasks have run, use
+// TaskScheduler::GetInstance()->FlushForTesting().
+//
+// When possible, use ScopedTaskScheduler instead of this. Tasks posted within
+// the scope of a ScopedTaskScheduler run synchronously, which makes tests more
+// deterministic.

[gab, 2017/01/09 19:55:46]

Does it really make them more deterministic? If the proper flushes are in place,
the tasks are still posted in order and I don't think it increases determinism.
It does make the tests simpler though because everything all side-effects
(posted tasks) from a task are resolved in a single block (rephrase to highlight
that instead?)

[fdoray, 2017/01/09 20:23:48]

Done.

Note: With a ScopedAsyncTaskScheduler, the order in which instructions from the
test body and instructions from asynchronous tasks run is not deterministic. It
is with a ScopedTaskScheduler.

+class ScopedAsyncTaskScheduler {
+ public:
+  // Registers a single-threaded TaskScheduler.
+  ScopedAsyncTaskScheduler();
+
+  // Shuts down and unregisters the TaskScheduler.
+  ~ScopedAsyncTaskScheduler();
+
+ private:
+  const TaskScheduler* task_scheduler_ = nullptr;
+
+  DISALLOW_COPY_AND_ASSIGN(ScopedAsyncTaskScheduler);
+};
+
+}  // namespace test
+}  // namespace base
+
+#endif  // BASE_TEST_SCOPED_ASYNC_TASK_SCHEDULER_H_