Use ScopedTaskEnvironment instead of MessageLoop in /skia/public/inte…

…rfaces/test/struct_traits_unittest.cc

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=rsesek@chromium.org

Change-Id: I844b1fed4483730544c9b03cd319968fcefad3c1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1648200
Auto-Submit: Carlos Caballero <carlscab@google.com>
Reviewed-by: Robert Sesek <rsesek@chromium.org>
Commit-Queue: Robert Sesek <rsesek@chromium.org>
Cr-Commit-Position: refs/heads/master@{#667639}

skia/public/interfaces/test/struct_traits_unittest.cc

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-#include "base/message_loop/message_loop.h"
+#include "base/test/scoped_task_environment.h"
 #include "mojo/public/cpp/bindings/binding_set.h"
 #include "skia/public/interfaces/test/traits_test_service.mojom.h"
 #include "testing/gtest/include/gtest/gtest.h"
@@ -46,7 +46,7 @@ class StructTraitsTest : public testing::Test, public mojom::TraitsTestService {
     std::move(callback).Run(t);
   }
 
-  base::MessageLoop loop_;
+  base::test::ScopedTaskEnvironment scoped_task_environment_;
   mojo::BindingSet<TraitsTestService> traits_test_bindings_;
 
   DISALLOW_COPY_AND_ASSIGN(StructTraitsTest);