Use ScopedTaskEnvironment instead of MessageLoopForUI in /ios/chrome/browser/ui/settings

MessageLoopForUI will go away, eventually.

ScopedTaskEnvironment will per default start a ThreadPool, which should
be fine in most of the cases. If you believe your test needs to make sure
that no ThreadPool runs let me know and I will update the patch.

BUG=891670
This CL was uploaded by git cl split.

R=sczs@chromium.org

Change-Id: Icff05dbcd5ffbea9419cd257203d739e5eb226f2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1718026
Auto-Submit: Carlos Caballero <carlscab@google.com>
Reviewed-by: Sergio Collazos <sczs@chromium.org>
Commit-Queue: Sergio Collazos <sczs@chromium.org>
Cr-Commit-Position: refs/heads/master@{#681897}

ios/chrome/browser/ui/settings/dataplan_usage_table_view_controller_unittest.mm

@@ -9,7 +9,7 @@
 #include "base/compiler_specific.h"
 #include "base/files/file_path.h"
 #include "base/memory/ref_counted.h"
-#include "base/message_loop/message_loop.h"
+#include "base/test/scoped_task_environment.h"
 #include "components/prefs/pref_registry_simple.h"
 #include "components/prefs/pref_service.h"
 #include "components/sync_preferences/pref_service_mock_factory.h"
@@ -57,7 +57,8 @@ class DataplanUsageTableViewControllerTest
 
     sync_preferences::PrefServiceMockFactory factory;
     base::FilePath path("DataplanUsageTableViewControllerTest.pref");
-    factory.SetUserPrefsFile(path, message_loop_.task_runner().get());
+    factory.SetUserPrefsFile(
+        path, scoped_task_environment_.GetMainThreadTaskRunner().get());
     return factory.Create(registry.get());
   }
 
@@ -70,7 +71,8 @@ class DataplanUsageTableViewControllerTest
     EXPECT_EQ(accessory_type, cell.accessoryType);
   }
 
-  base::MessageLoopForUI message_loop_;
+  base::test::ScopedTaskEnvironment scoped_task_environment_{
+      base::test::ScopedTaskEnvironment::MainThreadType::UI};
   std::unique_ptr<PrefService> pref_service_;
   DataplanUsageTableViewController* dataplanController_;
 };