We used to call StoreRegistration() immediately at the start of a register job. I removed this, though, because storing a registration for a service worker that may fail to install did not make sense. Doing this, though, introduced another bug. If we resurrect an existing registration by clearing its uninstalling flag, then we can now end up with a live registration that is not persisted to disk. We should call StoreRegistration() iff we flip mPendingUninstall from true to false in the register job.