[ServiceWorker] The user agents? (#774)

If there some reason why we talk about "user agents" in plural form?

If not, then maybe change: 

> The user agents must persistently keep a list of registered service worker registrations unless otherwise they are explicitly unregistered. 

To: 
> A user agent must persistently keep a list of registered service worker registrations, unless those registrations have been explicitly unregistered.  



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https://github.com/slightlyoff/ServiceWorker/issues/774