Converts WindowTreeClientTest to be in terms of aura

ui/aura/mus/window_port_mus.cc

Index: ui/aura/mus/window_port_mus.cc
diff --git a/ui/aura/mus/window_port_mus.cc b/ui/aura/mus/window_port_mus.cc
index d5a64e88c79980562c5792a140bbc36c9b926852..35fab977ef5be4b9841408de7b07407b71a8f81f 100644
--- a/ui/aura/mus/window_port_mus.cc
+++ b/ui/aura/mus/window_port_mus.cc
@@ -332,6 +332,10 @@ WindowPortMus::PrepareForServerVisibilityChange(bool value) {
   return std::move(data);
 }
 
+void WindowPortMus::PrepareForDestroy() {
+  ScheduleChange(ServerChangeType::DESTROY, ServerChangeData());
+}
+
 void WindowPortMus::PrepareForTransientRestack(WindowMus* window) {
   ServerChangeData change_data;
   change_data.child_id = window->server_id();