[w3c/push-api] Make PushSubscription.expirationTime mandatory (#302)

Push subscriptions can last many years without expiring and there is a negative correlation between the age of a push subscription and its expected delivery rate, as described by this study (that I have published):

https://blog.pushpad.xyz/2018/09/web-push-subscription-age-affects-delivery-rates/

Enforcing the browser to set an `expirationTime` for all the push subscription generated, would be helpful. Basically that will ensure that an unused endpoint will always be removed after some time... otherwise the number of old, unused endpoints will increase steadily over time (and the average delivery rate will tend toward zero).

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