Backport some tests in /ui/display 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
/ui/display.

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

This CL was uploaded by git cl split.

R=dnicoara@chromium.org

Bug: 891670
Change-Id: I3c182143dc21321d260af70e344f4dbe8c61704d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1786955
Auto-Submit: Gabriel Charette <gab@chromium.org>
Reviewed-by: Daniel Nicoara <dnicoara@chromium.org>
Commit-Queue: Daniel Nicoara <dnicoara@chromium.org>
Cr-Commit-Position: refs/heads/master@{#693699}

ui/display/manager/configure_displays_task_unittest.cc

@@ -48,7 +48,7 @@ class ConfigureDisplaysTaskTest : public testing::Test {
   }
 
  protected:
-  base::test::TaskEnvironment task_environment_;
+  base::test::SingleThreadTaskEnvironment task_environment_;
   ActionLogger log_;
   TestNativeDisplayDelegate delegate_;
 

ui/display/manager/display_configurator_unittest.cc

@@ -312,7 +312,7 @@ class DisplayConfiguratorTest : public testing::Test {
     return result;
   }
 
-  base::test::TaskEnvironment task_environment_;
+  base::test::SingleThreadTaskEnvironment task_environment_;
   TestStateController state_controller_;
   TestMirroringController mirroring_controller_;
   DisplayConfigurator configurator_;