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

ui/aura/mus/window_manager_delegate.h

Index: ui/aura/mus/window_manager_delegate.h
diff --git a/ui/aura/mus/window_manager_delegate.h b/ui/aura/mus/window_manager_delegate.h
index 447a7a028ad2be017c7ba30d2e283d789fb62173..f07d4644898f3d757d9417cc0f51df964ff92b7c 100644
--- a/ui/aura/mus/window_manager_delegate.h
+++ b/ui/aura/mus/window_manager_delegate.h
@@ -91,6 +91,9 @@ class AURA_EXPORT WindowManagerDelegate {
       const std::string& name,
       std::unique_ptr<std::vector<uint8_t>>* new_data) = 0;
 
+  // A client requested the modal type to be changed to |type|.
+  virtual void OnWmSetModalType(Window* window, ui::ModalType type) = 0;
+
   // A client requested to change focusibility of |window|. We currently assume
   // this always succeeds.
   virtual void OnWmSetCanFocus(Window* window, bool can_focus) = 0;