Backport some tests in /chrome/browser/chromeos/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
/chrome/browser/chromeos/arc.

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

This CL was uploaded by git cl split.

R=elijahtaylor@chromium.org

Bug: 891670
Change-Id: I5945428ec9b8eeeb0165da3b24cb52709d2961b7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1786741
Auto-Submit: Gabriel Charette <gab@chromium.org>
Reviewed-by: Elijah Taylor <elijahtaylor@chromium.org>
Commit-Queue: Elijah Taylor <elijahtaylor@chromium.org>
Cr-Commit-Position: refs/heads/master@{#693953}

chrome/browser/chromeos/arc/bluetooth/arc_bluetooth_bridge_unittest.cc

@@ -209,7 +209,7 @@ class ArcBluetoothBridgeTest : public testing::Test {
   std::unique_ptr<FakeBluetoothInstance> fake_bluetooth_instance_;
   std::unique_ptr<ArcBluetoothBridge> arc_bluetooth_bridge_;
   scoped_refptr<device::BluetoothAdapter> adapter_;
-  base::test::TaskEnvironment task_environment_;
+  base::test::SingleThreadTaskEnvironment task_environment_;
   base::RunLoop get_adapter_run_loop_;
 };