Backport some tests in /components/bookmarks 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
/components/bookmarks.

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

This CL was uploaded by git cl split.

Edit: Revert changes to BookmarkNodeDataTest and BookmarkUtilsTest
      which rely on base::ThreadPool.

R=sky@chromium.org

Bug: 891670
Change-Id: I8a37c3ecc469fcbbdc52b80724f5d8b073e30414
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1786903
Commit-Queue: Scott Violet <sky@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Auto-Submit: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#694840}

components/bookmarks/browser/bookmark_expanded_state_tracker_unittest.cc

@@ -32,7 +32,7 @@ class BookmarkExpandedStateTrackerTest : public testing::Test {
   void SetUp() override;
   void TearDown() override;
 
-  base::test::TaskEnvironment task_environment_;
+  base::test::SingleThreadTaskEnvironment task_environment_;
   TestingPrefServiceSimple prefs_;
   std::unique_ptr<BookmarkModel> model_;
 

components/bookmarks/browser/bookmark_utils_unittest.cc

@@ -72,7 +72,7 @@ class BookmarkUtilsTest : public testing::Test,
     ++grouped_changes_ended_count_;
   }
 
-  // Clipboard requires a message loop.
+  // Clipboard requires a full TaskEnvironment.
   base::test::TaskEnvironment task_environment_;
 
   int grouped_changes_beginning_count_;

components/bookmarks/managed/managed_bookmarks_tracker_unittest.cc

@@ -165,7 +165,7 @@ class ManagedBookmarksTrackerTest : public testing::Test {
   }
 
   base::ScopedTempDir scoped_temp_dir_;
-  base::test::TaskEnvironment task_environment_;
+  base::test::SingleThreadTaskEnvironment task_environment_;
   TestingPrefServiceSimple prefs_;
   std::unique_ptr<BookmarkModel> model_;
   MockBookmarkModelObserver observer_;