Context: https://chromium-review.googlesource.com/c/chromium/src/+/987832/2/content/browser/service_worker/service_worker_provider_host.h#235

A ptr to ControllerServiceWorker can be obtained from ServiceWorkerProviderHost::GetControllerServiceWorkerPtr() but the method doesn't guarantee that the controller is running. Currently it's callsite's responsibility to make sure that the worker is running, but it would be better to have a consistent way to make sure the worker is running.