Backport some tests in /device/vr 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
/device/vr.

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

This CL was uploaded by git cl split.

R=alcooper@chromium.org

Bug: 891670
Change-Id: Ib2a8e5bdbfa859777406af079f62760ce69b9b60
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1786686
Auto-Submit: Gabriel Charette <gab@chromium.org>
Reviewed-by: Alexander Cooper <alcooper@chromium.org>
Commit-Queue: Alexander Cooper <alcooper@chromium.org>
Cr-Commit-Position: refs/heads/master@{#693770}

device/vr/vr_device_base_unittest.cc

@@ -107,7 +107,7 @@ class VRDeviceTest : public testing::Test {
     return display_info;
   }
 
-  base::test::TaskEnvironment task_environment_;
+  base::test::SingleThreadTaskEnvironment task_environment_;
 
   DISALLOW_COPY_AND_ASSIGN(VRDeviceTest);
 };