[no_gravity]

From how I read it, this reduces the probability that the data gets deleted due to low disk space / garbage collection:

    Persistent storage can help protect
    critical data from eviction, and
    educe the chance of data loss.

Are you sure this keeps the data when the user deletes their browser data? To me it sounds like it does not:

    Persistent storage is not deleted by the browser,
    even if storage is running low. It will only be
    deleted if the user chooses to remove it via
    their site settings.