As far as Microsoft is concerned, they're large enough to fork Chromium if they wanted to, just like how Google forked Webkit into Blink. Every other organization that ships a Chromium-based browser is fully at Google's mercy; Google holds the keys to the kingdom: https://chromium.googlesource.com/chromium/src/
Microsoft maintained their own browser engine for decades, they had good reasons to drop them - I'm not even sure if there would be enough staff left there to actually be able to keep up with Chromium in a fork.
