Backport some tests in /chrome/utility/image_writer 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/utility/image_writer.

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

This CL was uploaded by git cl split.

R=haven@chromium.org

Bug: 891670
Change-Id: Id42c705ac4a5dba6785b770aa62c377b029e1faa
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1786968
Auto-Submit: Gabriel Charette <gab@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#695741}

chrome/utility/image_writer/image_writer_unittest.cc

@@ -55,7 +55,7 @@ class ImageWriterUtilityTest : public testing::Test {
   base::FilePath device_path_;
 
  private:
-  base::test::TaskEnvironment task_environment_;
+  base::test::SingleThreadTaskEnvironment task_environment_;
   base::ScopedTempDir temp_dir_;
 };