Backport some tests in /components/download 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
/components/download.

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

This CL was uploaded by git cl split.

R=qinmin@chromium.org

Bug: 891670
Change-Id: I1c9593e1468988a93183f1e29edda455a0c20690
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1787064
Auto-Submit: Gabriel Charette <gab@chromium.org>
Reviewed-by: Min Qin <qinmin@chromium.org>
Commit-Queue: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#694580}

components/download/internal/background_service/scheduler/device_status_listener_unittest.cc

@@ -146,7 +146,7 @@ class DeviceStatusListenerTest : public testing::Test {
   MockObserver mock_observer_;
 
   // Needed for network change notifier and power monitor.
-  base::test::TaskEnvironment task_environment_;
+  base::test::SingleThreadTaskEnvironment task_environment_;
   base::PowerMonitorTestSource* power_source_;
   TestBatteryStatusListener* test_battery_listener_;
 };