Backport some tests in /ui/events 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/events.

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

This CL was uploaded by git cl split.

R=sadrul@chromium.org

Bug: 891670
Change-Id: Iecc1db230e4c701499857b1376a721c845d13cc0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1785598
Auto-Submit: Gabriel Charette <gab@chromium.org>
Reviewed-by: Sadrul Chowdhury <sadrul@chromium.org>
Commit-Queue: Sadrul Chowdhury <sadrul@chromium.org>
Cr-Commit-Position: refs/heads/master@{#693764}

ui/events/platform/platform_event_source_unittest.cc

@@ -511,8 +511,8 @@ class PlatformEventTestWithMessageLoop : public PlatformEventTest {
   virtual void RunTestImpl() = 0;
 
  private:
-  base::test::TaskEnvironment task_environment_{
-      base::test::TaskEnvironment::MainThreadType::UI};
+  base::test::SingleThreadTaskEnvironment task_environment_{
+      base::test::SingleThreadTaskEnvironment::MainThreadType::UI};
 
   DISALLOW_COPY_AND_ASSIGN(PlatformEventTestWithMessageLoop);
 };