Back in January, we exclusively reported that Google wants to add Steam to Chrome OS and introduce more powerful Chromebooks, possibly running on AMD silicon. Now further details have emerged. 9to5Google found a new Linux emulator in the Google's Chromium Gerrit codenamed "Borealis" that includes a pre-installed copy of Steam. It might even replace the current Linux implementation in the long term.
Chrome OS has had a virtual Linux emulator in beta for more than a year, codenamed Crostini. It's not a full-blown separate OS but more of a collection of compatibility software that helps seamlessly integrate Linux apps with the rest of the Chrome OS interface. We can assume this is the model Borealis and the pre-installed Steam will adopt.
There's a significant difference between the two solutions, though: While Crostini is based on Debian, Borealis is a Ubuntu system, based on the long-term support version 18.04. The reason behind the shift is probably the company behind Steam, Valve. It developed Proton, a compatibility layer that makes games originally developed for Windows run on Linux. While the software technically works on any flavor of Linux, Valve recommends using Ubuntu.
Another code change in the Chromium Gerrit suggests that we'll first see Steam integration in Chromebooks with 10th-gen Intel Core processors, like Samsung's Galaxy Chromebook, the Asus Chromebook Flip C436, or the just-announced Acer Chromebook Spin 713. We don't know if existing devices are sure to receive support, though — the Crostini layer is also only compatible with a small selection of Chromebooks that launched before its introduction.
It's not impossible, but unlikely that Google will run and maintain two separate virtual Linux machines on its Chromebooks, so Crostini might be replaced by Borealis sooner rather than later. We can assume that Steam will also run on the AMD Ryzen-powered Chromebooks currently looming on the horizon.