Add system modals to the proper container in mus+ash.

ui/aura/test/aura_test_base.cc

Index: ui/aura/test/aura_test_base.cc
diff --git a/ui/aura/test/aura_test_base.cc b/ui/aura/test/aura_test_base.cc
index 6d7c340bac14bd51688b4127024bf9e4905c6571..3858cdbfad158dc11e14748dba66524dfc9205e3 100644
--- a/ui/aura/test/aura_test_base.cc
+++ b/ui/aura/test/aura_test_base.cc
@@ -169,6 +169,8 @@ bool AuraTestBase::OnWmSetProperty(
   return true;
 }
 
+void AuraTestBase::OnWmSetModalType(Window* window, ui::ModalType type) {}
+
 void AuraTestBase::OnWmSetCanFocus(Window* window, bool can_focus) {}
 
 Window* AuraTestBase::OnWmCreateTopLevelWindow(