Inform window manager about modal windows in mus+ash.

ui/aura/mus/window_tree_client.h

Index: ui/aura/mus/window_tree_client.h
diff --git a/ui/aura/mus/window_tree_client.h b/ui/aura/mus/window_tree_client.h
index 63073714c818f062032f27e0558111c8cbff5323..369b4fca474f94ba5b8444b5f40570f9bf7a75aa 100644
--- a/ui/aura/mus/window_tree_client.h
+++ b/ui/aura/mus/window_tree_client.h
@@ -29,6 +29,7 @@
 #include "ui/aura/mus/mus_types.h"
 #include "ui/aura/mus/window_manager_delegate.h"
 #include "ui/aura/mus/window_tree_host_mus_delegate.h"
+#include "ui/base/ui_base_types.h"
 
 namespace display {
 class Display;
@@ -391,6 +392,9 @@ class AURA_EXPORT WindowTreeClient
       Id window_id,
       const std::string& name,
       const base::Optional<std::vector<uint8_t>>& transit_data) override;
+  void WmSetModalType(uint32_t change_id,
+                      Id window_id,
+                      ui::ModalType type) override;
   void WmSetCanFocus(Id window_id, bool can_focus) override;
   void WmCreateTopLevelWindow(
       uint32_t change_id,