Backport some tests in /ash/system/message_center/arc to SingleThreadTaskEnvironment

These tests were previously migrated from single-threaded MessageLoop to
a multi-threaded TaskEnvironment (then named ScopedTaskEnvironment) as
part of crbug.com/891670.

//base OWNERS decided in retrospect that it was better to keep a
single-threaded option for TaskEnvironment and introduced
SingleThreadTaskEnvironment. This CL retrofits that decision for
/ash/system/message_center/arc.

This CL is a no-op if it passes CQ.

This CL was uploaded by git cl split.

R=yusukes@chromium.org

Bug: 891670
Change-Id: I46ca756b637ca39066abaf89c7ba8bde439a078c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1786750
Auto-Submit: Gabriel Charette <gab@chromium.org>
Reviewed-by: Yusuke Sato <yusukes@chromium.org>
Commit-Queue: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#694564}

ash/system/message_center/arc/arc_notification_manager_unittest.cc

@@ -167,7 +167,7 @@ class ArcNotificationManagerTest : public testing::Test {
     base::RunLoop().RunUntilIdle();
   }
 
-  base::test::TaskEnvironment task_environment_;
+  base::test::SingleThreadTaskEnvironment task_environment_;
   TestArcAppIdProvider app_id_provider_;
   std::unique_ptr<arc::FakeNotificationsInstance> arc_notifications_instance_;
   std::unique_ptr<mojo::Binding<arc::mojom::NotificationsInstance>> binding_;