Backport some tests in /ios/chrome/app/application_delegate 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
/ios/chrome/app/application_delegate.

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

This CL was uploaded by git cl split.

R=marq@chromium.org

Bug: 891670
Change-Id: If751e4e85e6468aef3eddf4420343fa30986f292
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1786904
Commit-Queue: Gabriel Charette <gab@chromium.org>
Auto-Submit: Gabriel Charette <gab@chromium.org>
Reviewed-by: Mark Cogan <marq@chromium.org>
Cr-Commit-Position: refs/heads/master@{#694584}

ios/chrome/app/application_delegate/memory_warning_helper_unittest.mm

@@ -52,8 +52,7 @@ class MemoryWarningHelperTest : public PlatformTest {
   void RunMessageLoop() { run_loop_.Run(); }
 
  private:
-  base::test::TaskEnvironment task_environment_{
-      base::test::TaskEnvironment::ThreadingMode::MAIN_THREAD_ONLY};
+  base::test::SingleThreadTaskEnvironment task_environment_;
   base::RunLoop run_loop_;
   base::MemoryPressureListener::MemoryPressureLevel memory_pressure_level_;
   std::unique_ptr<base::MemoryPressureListener> memory_pressure_listener_;