More context: https://chromium-review.googlesource.com/c/chromium/src/+/912051/16/content/browser/loader/prefetch_url_loader_factory.cc#24

The bind calls to the interfaces added to BindRegistry are supposed to come on the IO thread unless the task runner is overriden, but in the case above (PrefetchURLLoaderFactory::ConnectToService) it seems it comes on non-IO thread in content_browsertests (more specifically, content/browser/loader/prefetch_browsertest.cc in this particular case).

I haven't checked if the same happens in other tests or not, but some Bind calls apparently have additional post tasks to the IO thread, so we'd better clarify this.