Use ScopedTaskEnvironment instead of MessageLoop in /ash

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=afakhry@chromium.org

Change-Id: I089353dab84610c892f13338e3de419caddf0b37
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1649508
Auto-Submit: Carlos Caballero <carlscab@google.com>
Reviewed-by: Ahmed Fakhry <afakhry@chromium.org>
Commit-Queue: Ahmed Fakhry <afakhry@chromium.org>
Cr-Commit-Position: refs/heads/master@{#667214}

ash/public/cpp/default_scale_factor_retriever_unittest.cc

@@ -6,8 +6,8 @@
 
 #include "ash/public/interfaces/cros_display_config.mojom.h"
 #include "base/bind.h"
-#include "base/message_loop/message_loop.h"
 #include "base/run_loop.h"
+#include "base/test/scoped_task_environment.h"
 #include "mojo/public/cpp/bindings/binding.h"
 #include "testing/gtest/include/gtest/gtest.h"
 #include "ui/display/display.h"
@@ -73,7 +73,7 @@ class DefaultScaleFactorRetrieverTest : public testing::Test {
   ~DefaultScaleFactorRetrieverTest() override = default;
 
  private:
-  base::MessageLoop message_loop_;
+  base::test::ScopedTaskEnvironment scoped_task_environment_;
   DISALLOW_COPY_AND_ASSIGN(DefaultScaleFactorRetrieverTest);
 };