aura-mus: Implement stacking in DesktopWindowTreeHostMus.

ui/aura/mus/window_tree_client.cc

Index: ui/aura/mus/window_tree_client.cc
diff --git a/ui/aura/mus/window_tree_client.cc b/ui/aura/mus/window_tree_client.cc
index 13660c96943379fbde45e8063fb00f656942a44e..d8c4c4755190be2885fa6995dda3ac7ffe97cf08 100644
--- a/ui/aura/mus/window_tree_client.cc
+++ b/ui/aura/mus/window_tree_client.cc
@@ -1283,6 +1283,8 @@ void WindowTreeClient::OnChangeCompleted(uint32_t change_id, bool success) {
   if (!change)
     return;
 
+  delegate_->OnChangeCompleted(change->change_type(), success);
+
   if (!success)
     change->ChangeFailed();
 
@@ -1610,6 +1612,14 @@ void WindowTreeClient::OnWindowTreeHostDeactivateWindow(
       WindowMus::Get(window_tree_host->window())->server_id());
 }
 
+void WindowTreeClient::OnWindowTreeHostStackAtTop(
+    WindowTreeHostMus* window_tree_host) {
+  WindowMus* window = WindowMus::Get(window_tree_host->window());
+  const uint32_t change_id = ScheduleInFlightChange(
+      base::MakeUnique<CrashInFlightChange>(window, ChangeType::REORDER));
+  tree_->StackAtTop(change_id, window->server_id());
+}
+
 std::unique_ptr<WindowPortMus> WindowTreeClient::CreateWindowPortForTopLevel(
     const std::map<std::string, std::vector<uint8_t>>* properties) {
   std::unique_ptr<WindowPortMus> window_port =