Backport some tests in /chrome/browser/prefs 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/prefs.

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

This CL was uploaded by git cl split.

R=battre@chromium.org

Bug: 891670
Change-Id: I8e1135fec9f4a504865151a97ce730532f030228
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1787077
Auto-Submit: Gabriel Charette <gab@chromium.org>
Reviewed-by: Dominic Battré <battre@chromium.org>
Commit-Queue: Dominic Battré <battre@chromium.org>
Cr-Commit-Position: refs/heads/master@{#693606}

chrome/browser/prefs/chrome_command_line_pref_store_ssl_manager_unittest.cc

@@ -22,7 +22,7 @@ class CommandLinePrefStoreSSLManagerTest : public testing::Test {
   CommandLinePrefStoreSSLManagerTest() {}
 
  protected:
-  base::test::TaskEnvironment task_environment_;
+  base::test::SingleThreadTaskEnvironment task_environment_;
 };
 
 // Test that command-line settings for SSL versions are respected and that they

chrome/browser/prefs/profile_pref_store_manager_unittest.cc

@@ -326,7 +326,7 @@ class ProfilePrefStoreManagerTest : public testing::Test,
       ADD_FAILURE() << "No validation observed for preference: " << pref_path;
   }
 
-  base::test::TaskEnvironment task_environment_;
+  base::test::SingleThreadTaskEnvironment task_environment_;
   std::vector<prefs::mojom::TrackedPreferenceMetadataPtr> configuration_;
   base::ScopedTempDir profile_dir_;
   scoped_refptr<user_prefs::PrefRegistrySyncable> profile_pref_registry_;