Thanks Adams3141 and Verlane.
I am starting to agree with both of you that part of what is causing this problem is something about Chrome the way it is set up on my computer.
But part of it must have to do with OGS, too. This has happened to me occasionally before I started using OGS, but rarely, and not for a long time. I played a lot of games on OGS in the last four days of 2017, and then I deleted my browsing history, etc., and got Chrome to stop sending me that message. I didn’t come back to OGS for a week, but I used Chrome many times that week with no problem. Then one day I poked around in OGS several times, and that same day I started getting the messages from Chrome again.
So I guess there’s something about Chrome as it is set up on my computer that makes it have a problem with something about the OGS website that it doesn’t encounter on any other website.
I was hoping and expecting that several other go players on OGS who use Chrome would have had the same problem and maybe figured out a good way to avoid it, but it doesn’t look like anyone else has had this problem. At least for now, I want to avoid doing anything drastic like reinstalling Chrome or switching to another browser (I already switched from IE to Chrome), so I guess I’ll just put up with it for a while, deleting my browsing history, etc., every so often, unless someone here who has had this problems suggests what I should do.