Inform window manager about modal windows 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 8a0f04bdae6cd244bdb002276aa6283494dab800..e704c5d8b089bc580106aeb6c77f36420f9afc95 100644
--- a/ui/aura/mus/window_manager_delegate.h
+++ b/ui/aura/mus/window_manager_delegate.h
@@ -91,6 +91,10 @@ class AURA_EXPORT WindowManagerDelegate {
       const std::string& name,
       std::unique_ptr<std::vector<uint8_t>>* new_data) = 0;
 
+  // A client requested modal type of |window| changed to |type|. Return true to
+  // allow the change to |type|, false otherwise.
+  virtual bool 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;