aura-mus: Implement Deactivate().

ui/aura/test/mus/test_window_tree.cc

Index: ui/aura/test/mus/test_window_tree.cc
diff --git a/ui/aura/test/mus/test_window_tree.cc b/ui/aura/test/mus/test_window_tree.cc
index f32e28e4777b3af0ca9f752c35a44026075c6237..aebfee306e74eca029fccf22c9426df7a14995ff 100644
--- a/ui/aura/test/mus/test_window_tree.cc
+++ b/ui/aura/test/mus/test_window_tree.cc
@@ -266,6 +266,8 @@ void TestWindowTree::OnWindowInputEventAck(uint32_t event_id,
   acked_events_.push_back({event_id, result});
 }
 
+void TestWindowTree::DeactivateWindow(uint32_t window_id) {}
+
 void TestWindowTree::GetWindowManagerClient(
     mojo::AssociatedInterfaceRequest<ui::mojom::WindowManagerClient> internal) {
 }