Inform window manager about modal windows in mus+ash.

services/ui/demo/mus_demo_internal.cc

Index: services/ui/demo/mus_demo_internal.cc
diff --git a/services/ui/demo/mus_demo_internal.cc b/services/ui/demo/mus_demo_internal.cc
index 6c2125692fa142f61cc57ce1945e241549a8c72f..88c441086a3550541fa6f6aef9f16bd30d2cbd6c 100644
--- a/services/ui/demo/mus_demo_internal.cc
+++ b/services/ui/demo/mus_demo_internal.cc
@@ -48,6 +48,11 @@ bool MusDemoInternal::OnWmSetProperty(
   return true;
 }
 
+bool MusDemoInternal::OnWmSetModalType(aura::Window* window,
+                                       ui::ModalType type) {
+  return true;
+}
+
 void MusDemoInternal::OnWmSetCanFocus(aura::Window* window, bool can_focus) {}
 
 aura::Window* MusDemoInternal::OnWmCreateTopLevelWindow(