Use ScopedTaskEnvironment instead of MessageLoop in /chrome/utility/image_writer

MessageLoop will go away, eventually.

ScopedTaskEnvironment will per default start a ThreadPool, which should
be fine in most of the cases. If you belive 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=haven@chromium.org

Change-Id: I66ca154ecb0432c3843515347ddfdd39dad6f3c8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1649510
Auto-Submit: Carlos Caballero <carlscab@google.com>
Commit-Queue: Carlos Caballero <carlscab@google.com>
Commit-Queue: Vasilii Sukhanov <vasilii@chromium.org>
Reviewed-by: Vasilii Sukhanov <vasilii@chromium.org>
Cr-Commit-Position: refs/heads/master@{#682664}

chrome/utility/image_writer/image_writer_unittest.cc

@@ -9,8 +9,8 @@
 
 #include "base/files/file_util.h"
 #include "base/files/scoped_temp_dir.h"
-#include "base/message_loop/message_loop.h"
 #include "base/run_loop.h"
+#include "base/test/scoped_task_environment.h"
 #include "chrome/utility/image_writer/error_messages.h"
 #include "chrome/utility/image_writer/image_writer.h"
 #include "chrome/utility/image_writer/image_writer_handler.h"
@@ -55,7 +55,7 @@ class ImageWriterUtilityTest : public testing::Test {
   base::FilePath device_path_;
 
  private:
-  base::MessageLoop message_loop_;
+  base::test::ScopedTaskEnvironment scoped_task_environment_;
   base::ScopedTempDir temp_dir_;
 };