Inform window manager about modal windows in mus+ash.

ash/mus/window_manager.cc

Index: ash/mus/window_manager.cc
diff --git a/ash/mus/window_manager.cc b/ash/mus/window_manager.cc
index 8818e3cbdd393d96889b57860a3bdf4eaa865c69..18864effade5fdb78a004fa051e8072107481203 100644
--- a/ash/mus/window_manager.cc
+++ b/ash/mus/window_manager.cc
@@ -38,6 +38,7 @@
 #include "services/ui/public/cpp/property_type_converters.h"
 #include "services/ui/public/interfaces/constants.mojom.h"
 #include "services/ui/public/interfaces/window_manager.mojom.h"
+#include "ui/aura/client/aura_constants.h"
 #include "ui/aura/client/window_parenting_client.h"
 #include "ui/aura/env.h"
 #include "ui/aura/mus/capture_synchronizer.h"
@@ -321,6 +322,11 @@ bool WindowManager::OnWmSetProperty(
   return false;
 }
 
+bool WindowManager::OnWmSetModalType(aura::Window* window, ui::ModalType type) {
+  window->SetProperty(aura::client::kModalKey, type);
+  return true;
+}
+
 void WindowManager::OnWmSetCanFocus(aura::Window* window, bool can_focus) {
   NonClientFrameController* non_client_frame_controller =
       NonClientFrameController::Get(window);