This seems to be common, and after a bit of searching (after it first happened to my wife's 6p) there is an easy fix. Go into Google settings on your phone, Ads, and then select 'Reset marketing ID'. The thinking is that this fake virus screen is somehow linked to Google advertising ID, and that you'll keep being served with the same 'advert' when you visit the same site, regardless of whether you clear your cache, reset your phone etc, until you reset your advertising ID (after which it doesn't follow you).
Oddly, I had the same thing happen to me not long after getting my S7. I reset my Advertising ID, and (like my wife with her 6p) the problem appears to have stopped (and this was multiple weeks ago).
You'll find this suggestion elsewhere in other forums too, but not before wading through a whole lot of well meaning 'clear the cache, reset the phone' etc suggestions.
Hope this helps!
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