Mojo C++ bindings: reduce references to AssociatedGroupController.

mojo/public/cpp/bindings/associated_binding.h

Index: mojo/public/cpp/bindings/associated_binding.h
diff --git a/mojo/public/cpp/bindings/associated_binding.h b/mojo/public/cpp/bindings/associated_binding.h
index 05d80f3c740c65bc3911fba7a6f49f9b1288125e..5c3764f5ee672b1518409cab20e80809a5ca3b2e 100644
--- a/mojo/public/cpp/bindings/associated_binding.h
+++ b/mojo/public/cpp/bindings/associated_binding.h
@@ -110,8 +110,10 @@ class AssociatedBinding {
         base::WrapUnique(new typename Interface::RequestValidator_()),
         Interface::HasSyncMethods_, std::move(runner), Interface::Version_));
 
-    stub_.serialization_context()->group_controller =
-        endpoint_client_->group_controller();
+    if (Interface::PassesAssociatedKinds_) {
+      stub_.serialization_context()->group_controller =
+          endpoint_client_->group_controller();
+    }
   }
 
   // Adds a message filter to be notified of each incoming message before