Inform window manager about modal windows 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..d3da04dbb8943405c647edf253993f5f9a60c19b 100644
--- a/ui/aura/test/aura_test_base.cc
+++ b/ui/aura/test/aura_test_base.cc
@@ -169,6 +169,10 @@ bool AuraTestBase::OnWmSetProperty(
   return true;
 }
 
+bool AuraTestBase::OnWmSetModalType(aura::Window* window, ui::ModalType type) {
+  return true;
+}
+
 void AuraTestBase::OnWmSetCanFocus(Window* window, bool can_focus) {}
 
 Window* AuraTestBase::OnWmCreateTopLevelWindow(