Need to spec out how Page Lifecycle interacts with Wake Lock

[tomayac]

It is currently unclear how Page Lifecycle interacts with Wake Lock. This is the reciprocal issue to https://github.com/w3c/wake-lock/issues/139.

[spanicker]

This should be easy change for Page lifecycle spec. i.e. updating the guidance on when to avoid freezing (see second note here):
https://wicg.github.io/page-lifecycle/spec.html#frozenness-state
Though it is to the UA's discretion as to what exclusions to make.