So it shouldn't be "Whatsapp" for something. But "Something" for Whatsapp. I'm not entirely sure if that avoids all the legal issues or whether it was just something Reddit itself was willing to compromise on in the case of "Relay for Reddit", but at least you should start with that kind of default rather than shamelessly using other company's product name as if it was your own.
I think others who have used Google's name or other product names in their own Chrome extensions have taken a similar path. Like how "Gmail Checker Plus" (or something) became "Checker Plus For Gmail" [2].
[1] - https://play.google.com/store/apps/details?id=free.reddit.ne...