mus: Allow the WM to specify the windows that can have active children.

components/mus/public/cpp/tests/window_server_test_base.h

Index: components/mus/public/cpp/tests/window_server_test_base.h
diff --git a/components/mus/public/cpp/tests/window_server_test_base.h b/components/mus/public/cpp/tests/window_server_test_base.h
index 0bb6cda35675595e93c484b7713b3fd02bd6407e..03d905da69bb2df910c445eeeae313d3c20e908c 100644
--- a/components/mus/public/cpp/tests/window_server_test_base.h
+++ b/components/mus/public/cpp/tests/window_server_test_base.h
@@ -45,6 +45,7 @@ class WindowServerTestBase
   WindowTreeConnection* window_manager() { return window_manager_; }
 
  protected:
+  mojom::WindowTreeHost* host() { return host_.get(); }
   WindowTreeConnection* most_recent_connection() {
     return most_recent_connection_;
   }