Fixes bug in handling restacking because of transients

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 6ae11599f15d21690585153600802a395c955670..abb468e95c886b54d3deb1121b951267734a5a0f 100644
--- a/ui/aura/mus/window_tree_client.cc
+++ b/ui/aura/mus/window_tree_client.cc
@@ -1550,6 +1550,22 @@ void WindowTreeClient::OnTransientChildWindowRemoved(Window* parent,
   tree_->RemoveTransientWindowFromParent(change_id, child_mus->server_id());
 }
 
+void WindowTreeClient::OnWillRestackTransientChildAbove(Window* source,
+                                                        Window* window) {
+  DCHECK(source->parent());
+  DCHECK_EQ(source->parent(), window->parent());
+  WindowMus::Get(source->parent())
+      ->PrepareForTransientRestack(WindowMus::Get(window));
+}
+
+void WindowTreeClient::OnDidRestackTransientChildAbove(Window* source,
+                                                       Window* window) {
+  DCHECK(source->parent());
+  DCHECK_EQ(source->parent(), window->parent());
+  WindowMus::Get(source->parent())
+      ->OnTransientRestackDone(WindowMus::Get(window));
+}
+
 uint32_t WindowTreeClient::CreateChangeIdForDrag(WindowMus* window) {
   return ScheduleInFlightChange(
       base::MakeUnique<InFlightDragChange>(window, ChangeType::DRAG_LOOP));