Backport some tests in...

Backport some tests in /device/gamepad/public/cpp/gamepad_mojom_traits_unittest.cc 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/gamepad/public/cpp/gamepad_mojom_traits_unittest.cc.

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

This CL was uploaded by git cl split.

R=estark@chromium.org

Bug: 891670
Change-Id: Ic5c828e2748caa0f834ac8a6dab5693e1394cb4b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1787202
Auto-Submit: Gabriel Charette <gab@chromium.org>
Reviewed-by: Emily Stark <estark@chromium.org>
Commit-Queue: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#695136}

device/gamepad/public/cpp/gamepad_mojom_traits_unittest.cc

@@ -164,7 +164,7 @@ class GamepadStructTraitsTest : public testing::Test {
   GamepadStructTraitsTest() {}
 
  private:
-  base::test::TaskEnvironment task_environment_;
+  base::test::SingleThreadTaskEnvironment task_environment_;
 
   DISALLOW_COPY_AND_ASSIGN(GamepadStructTraitsTest);
 };