Backport some tests in /chrome/browser/ui/views/frame 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
/chrome/browser/ui/views/frame.

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

This CL was uploaded by git cl split.

R=sky@chromium.org

Bug: 891670
Change-Id: If7dc376b444ed8357e741cb7505ac480cc2bc51d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1786818
Auto-Submit: Gabriel Charette <gab@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#694566}

chrome/browser/ui/views/frame/web_contents_close_handler_unittest.cc

@@ -13,7 +13,8 @@ class MockWebContentsCloseHandlerDelegate
     : public WebContentsCloseHandlerDelegate {
  public:
   MockWebContentsCloseHandlerDelegate()
-      : task_environment_(base::test::TaskEnvironment::MainThreadType::UI),
+      : task_environment_(
+            base::test::SingleThreadTaskEnvironment::MainThreadType::UI),
         got_clone_(false),
         got_destroy_(false) {}
   ~MockWebContentsCloseHandlerDelegate() override {}
@@ -33,7 +34,7 @@ class MockWebContentsCloseHandlerDelegate
   void DestroyClonedLayer() override { got_destroy_ = true; }
 
  private:
-  base::test::TaskEnvironment task_environment_;
+  base::test::SingleThreadTaskEnvironment task_environment_;
   bool got_clone_;
   bool got_destroy_;