Backport some tests in /ui/views 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/views.

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

This CL was uploaded by git cl split.

Edit: Restore full TaskEnvironment in ViewsTestBase.

R=weili@chromium.org

Bug: 891670
Change-Id: Id7d806ac0dcacb9bdeeb3e7117c659ffbb46c217
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1786953Reviewed-by: Wei Li <weili@chromium.org>
Commit-Queue: Gabriel Charette <gab@chromium.org>
Auto-Submit: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#694884}

ui/views/animation/bounds_animator_unittest.cc

@@ -77,7 +77,8 @@ class TestView : public View {
 class BoundsAnimatorTest : public testing::Test {
  public:
   BoundsAnimatorTest()
-      : task_environment_(base::test::TaskEnvironment::MainThreadType::UI),
+      : task_environment_(
+            base::test::SingleThreadTaskEnvironment::MainThreadType::UI),
         child_(new TestView()),
         animator_(&parent_) {
     parent_.AddChildView(child_);
@@ -89,7 +90,7 @@ class BoundsAnimatorTest : public testing::Test {
   BoundsAnimator* animator() { return &animator_; }
 
  private:
-  base::test::TaskEnvironment task_environment_;
+  base::test::SingleThreadTaskEnvironment task_environment_;
   TestView parent_;
   TestView* child_;  // Owned by |parent_|.
   BoundsAnimator animator_;