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..8542b064cc2b778814d74160f0a03df8aa07c1c8 100644
--- a/ui/aura/mus/window_tree_client.cc
+++ b/ui/aura/mus/window_tree_client.cc
@@ -1550,6 +1550,24 @@ void WindowTreeClient::OnTransientChildWindowRemoved(Window* parent,
   tree_->RemoveTransientWindowFromParent(change_id, child_mus->server_id());
 }
 
+void WindowTreeClient::OnWillRestackTransientChildAbove(
+    Window* parent,
+    Window* transient_child) {
+  DCHECK(parent->parent());
+  DCHECK_EQ(parent->parent(), transient_child->parent());
+  WindowMus::Get(parent->parent())
+      ->PrepareForTransientRestack(WindowMus::Get(transient_child));
+}
+
+void WindowTreeClient::OnDidRestackTransientChildAbove(
+    Window* parent,
+    Window* transient_child) {
+  DCHECK(parent->parent());
+  DCHECK_EQ(parent->parent(), transient_child->parent());
+  WindowMus::Get(parent->parent())
+      ->OnTransientRestackDone(WindowMus::Get(transient_child));
+}
+
 uint32_t WindowTreeClient::CreateChangeIdForDrag(WindowMus* window) {
   return ScheduleInFlightChange(
       base::MakeUnique<InFlightDragChange>(window, ChangeType::DRAG_LOOP));