TopSites has a concept of "forced URLs" that it keeps track of. These are URLs that aren't part of the actual TopSites, but for which we still want to capture and store thumbnails. In practice, this is used to get thumbnails for MostLikely suggestions - these are added as forced URLs to TopSites when they show up on the NTP.

Since  bug 514752  (launched in M60), MostLikely suggestions come through Chrome's SuggestionsService, so the whole forced URLs hack isn't necessary anymore - we can just ask the SuggestionsService directly. In particular, TopSites::IsKnownURL would need to check SuggestionsService in addition to its own known URLs.