Backport some tests in /ui/events/blink 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/blink.

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

This CL was uploaded by git cl split.

R=bokan@chromium.org

Bug: 891670
Change-Id: I626ba137031b1d510206f119075f23024408141a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1786940
Auto-Submit: Gabriel Charette <gab@chromium.org>
Reviewed-by: David Bokan <bokan@chromium.org>
Commit-Queue: David Bokan <bokan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#693756}

ui/events/blink/input_handler_proxy_unittest.cc

@@ -518,7 +518,7 @@ class InputHandlerProxyEventQueueTest : public testing::Test {
 
   uint64_t next_begin_frame_number_ = viz::BeginFrameArgs::kStartingFrameNumber;
 
-  base::test::TaskEnvironment task_environment_;
+  base::test::SingleThreadTaskEnvironment task_environment_;
   base::WeakPtrFactory<InputHandlerProxyEventQueueTest> weak_ptr_factory_{this};
 };