Add base::test::ScopedTaskEnvironment.

base/test/scoped_task_environment.h

Index: base/test/scoped_task_environment.h
diff --git a/base/test/scoped_task_environment.h b/base/test/scoped_task_environment.h
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+++ b/base/test/scoped_task_environment.h
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+// 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_TASK_ENVIRONMENT_H_
+#define BASE_TEST_SCOPED_TASK_ENVIRONMENT_H_
+
+#include "base/macros.h"
+#include "base/message_loop/message_loop.h"
+
+namespace base {
+
+class TaskScheduler;
+
+namespace test {
+
+// ScopedTaskEnvironment allows usage of these APIs within its scope:
+// - (Thread|Sequenced)TaskRunnerHandle, on the thread where it lives
+// - base/task_scheduler/post_task.h, on any thread
+//
+// Tests that need either of these APIs should instantiate a
+// ScopedTaskEnvironment.
+//
+// Tasks posted to the (Thread|Sequenced)TaskRunnerHandle run synchronously when
+// RunLoop::Run(UntilIdle) is called on the thread where the
+// ScopedTaskEnvironment lives.
+//
+// Tasks posted through base/task_scheduler/post_task.h run on dedicated threads
+// as they are posted.
+//
+// Design and future improvements documented in
+// https://docs.google.com/document/d/1QabRo8c7D9LsYY3cEcaPQbOCLo8Tu-6VLykYXyl3Pkk/edit
+class ScopedTaskEnvironment {
+ public:
+  ScopedTaskEnvironment();
+
+  // Runs pending (Thread|Sequenced)TaskRunnerHandle tasks and pending
+  // BLOCK_SHUTDOWN TaskScheduler tasks. Then, unregisters the TaskScheduler and
+  // the (Thread|Sequenced)TaskRunnerHandle.
+  ~ScopedTaskEnvironment();
+
+ private:
+  // Note: |message_loop_| is an implementation detail and will be replaced in
+  // the future, do NOT rely on the presence of a MessageLoop beyond
+  // (Thread|Sequenced)TaskRunnerHandle and RunLoop.
+  MessageLoop message_loop_;
+
+  const TaskScheduler* task_scheduler_ = nullptr;
+
+  DISALLOW_COPY_AND_ASSIGN(ScopedTaskEnvironment);
+};
+
+}  // namespace test
+}  // namespace base
+
+#endif  // BASE_TEST_SCOPED_ASYNC_TASK_SCHEDULER_H_