Allow ScopedTaskEnvironment to pump UI or IO messages on the main thread.

base/test/scoped_task_environment.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_TASK_ENVIRONMENT_H_
 #define BASE_TEST_SCOPED_TASK_ENVIRONMENT_H_
 
 #include "base/macros.h"
 #include "base/message_loop/message_loop.h"
 
@@ skipping 14 matching lines @@
 // 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();
+  enum class MainThreadType {
+    // The main thread doesn't pump messages.

[gab, 2017/04/27 14:22:59]

s/doesn't pump messages/doesn't pump system messages/ ?

+    DEFAULT,
+    // The main thread pumps UI messages.
+    UI,
+    // The main thread pumps asynchronous IO messages.
+    IO,
+  };
+
+  ScopedTaskEnvironment(
+      MainThreadType main_thread_type = MainThreadType::DEFAULT);
 
   // 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_