mus: Fix creating the factory for window-tree-client in renderer.

content/renderer/mus/render_widget_window_tree_client_factory.cc

Index: content/renderer/mus/render_widget_window_tree_client_factory.cc
diff --git a/content/renderer/mus/render_widget_window_tree_client_factory.cc b/content/renderer/mus/render_widget_window_tree_client_factory.cc
index e2a19987a0187c1cd1cd7bc116b59b8eeabc5e36..6292da4926691d31929f4d4d8b86c772e84cdfce 100644
--- a/content/renderer/mus/render_widget_window_tree_client_factory.cc
+++ b/content/renderer/mus/render_widget_window_tree_client_factory.cc
@@ -31,6 +31,7 @@ namespace {
 void BindMusConnectionOnMainThread(
     uint32_t routing_id,
     ui::mojom::WindowTreeClientRequest request) {
+  RendererWindowTreeClient::CreateIfNecessary(routing_id);
   RendererWindowTreeClient::Get(routing_id)->Bind(std::move(request));
 }