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

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

This CL was uploaded by git cl split.

R=nzolghadr@chromium.org

Bug: 891670
Change-Id: I92843b289dfb872d21b738caf86b7abf07c34db9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1787204
Auto-Submit: Gabriel Charette <gab@chromium.org>
Commit-Queue: Timothy Dresser <tdresser@chromium.org>
Reviewed-by: Timothy Dresser <tdresser@chromium.org>
Cr-Commit-Position: refs/heads/master@{#695152}

ui/events/gestures/gesture_provider_aura_unittest.cc

@@ -17,7 +17,8 @@ class GestureProviderAuraTest : public testing::Test,
                                 public GestureProviderAuraClient {
  public:
   GestureProviderAuraTest()
-      : task_environment_(base::test::TaskEnvironment::MainThreadType::UI) {}
+      : task_environment_(
+            base::test::SingleThreadTaskEnvironment::MainThreadType::UI) {}
 
   ~GestureProviderAuraTest() override {}
 
@@ -36,7 +37,7 @@ class GestureProviderAuraTest : public testing::Test,
  private:
   std::unique_ptr<GestureConsumer> consumer_;
   std::unique_ptr<GestureProviderAura> provider_;
-  base::test::TaskEnvironment task_environment_;
+  base::test::SingleThreadTaskEnvironment task_environment_;
 };
 
 TEST_F(GestureProviderAuraTest, IgnoresExtraPressEvents) {