Chrome randomly quits and Safari opens by itself

A few days ago I picked up what I believe to be adware. (A "friend" recommended a "reliable" website.)

I have a 2017 MacBook Pro running Sierra 10.12.6 and I always use Google Chrome (version 60.0.3112.101) for browsing. This adware (or whatever it is) causes Chrome to quit unexpectedly and open Safari (version 10.1.2) with a pop-up (see attached screen shot). I never acknowledge the pop-up and always force quit Safari.

Safari has no extensions or add-ons and I have cleared browsing data and cookies.

Since this began, I have installed free versions of Sophos and Malwarebytes and ran a scan with each. Both programs detected adware and claimed to remove it, but I still had the problem.

Last night I bit the bullet and purchased a year's subscription to Norton. (I have since uninstalled Sophos and Malwarebytes.) I ran a full scan and Norton found some adware and Trojans. Just now I had the same problem. What can I do to resolve this?