Use ScopedTaskEnvironment instead of MessageLoopForUI in ui/aura/test/.

ui/snapshot/snapshot_aura_unittest.cc

 // Copyright (c) 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "ui/snapshot/snapshot.h"
 
 #include <stddef.h>
 #include <stdint.h>
 
 #include "base/bind.h"
@@ skipping 86 matching lines @@
 
     // The ContextFactory must exist before any Compositors are created.
     // Snapshot test tests real drawing and readback, so needs pixel output.
     bool enable_pixel_output = true;
     ui::ContextFactory* context_factory = nullptr;
     ui::ContextFactoryPrivate* context_factory_private = nullptr;
 
     ui::InitializeContextFactoryForTests(enable_pixel_output, &context_factory,
                                          &context_factory_private);
 
-    helper_.reset(
-        new aura::test::AuraTestHelper(base::MessageLoopForUI::current()));
+    helper_.reset(new aura::test::AuraTestHelper());
     helper_->SetUp(context_factory, context_factory_private);
     new ::wm::DefaultActivationClient(helper_->root_window());
   }
 
   void TearDown() override {
     test_window_.reset();
     delegate_.reset();
     helper_->RunAllPendingInMessageLoop();
     helper_->TearDown();
     ui::TerminateContextFactoryForTests();
@@ skipping 167 matching lines @@
   gfx::SizeF snapshot_size(test_bounds.size());
   snapshot_size.Scale(2.0f / kUIScale);
 
   gfx::Image snapshot = GrabSnapshotForTestWindow();
   EXPECT_EQ(gfx::ToRoundedSize(snapshot_size).ToString(),
             snapshot.Size().ToString());
   EXPECT_EQ(0u, GetFailedPixelsCountWithScaleFactor(snapshot, 2 / kUIScale));
 }
 
 }  // namespace ui