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

ui/aura/test/aura_test_helper.h

Index: ui/aura/test/aura_test_helper.h
diff --git a/ui/aura/test/aura_test_helper.h b/ui/aura/test/aura_test_helper.h
index 307b8002b8a6d9c4295cc31668843ae83295c096..25f612bb5e2d8ffab3db08b9d37a44533928951b 100644
--- a/ui/aura/test/aura_test_helper.h
+++ b/ui/aura/test/aura_test_helper.h
@@ -12,10 +12,6 @@
 #include "ui/aura/window_event_dispatcher.h"
 #include "ui/aura/window_tree_host.h"
 
-namespace base {
-class MessageLoopForUI;
-}
-
 namespace ui {
 class ContextFactory;
 class ScopedAnimationDurationScaleMode;
@@ -48,7 +44,7 @@ class TestWindowParentingClient;
 // that are necessary to run test on Aura.
 class AuraTestHelper {
  public:
-  explicit AuraTestHelper(base::MessageLoopForUI* message_loop);
+  AuraTestHelper();
   ~AuraTestHelper();
 
   // Returns the current AuraTestHelper, or nullptr if it's not alive.