The key holds a list of values representing applications along with trigger dates, after which you'll get asked again to install Chrome. Other ones I see there include Irfanview and Avast. I tested it by changing the date for "Piriform Ltd" to a past date, and then re-installing CCleaner. This time I got asked about Chrome.
So, one mystery solved, I guess. The only other thing I wonder about is the claim (by the same guy I mentioned in my first post) that CCleaner's offer to install Chrome could not be declined, that Chrome got installed no matter what you did. That was not my experience, but maybe there was a defective installer in the past. I did find one other similar report after doing a Google search, so maybe there's a grain of truth to the story, even if there's more to it than just that.