What does this even mean. Chromium or Webkit are not "native" to an OS. OSs don't magically include browser engines, its not a critical component of an OS either.
Most OSs do come with browsers preinstalled, but they are programs just like any other. You can remove Safari from macOS (albeit its pretty hard because root is read only and signed), you can remove Edge from Windows. In my desktop with Windows 10 the only browser I have is Firefox (not even Edge), does that make Gecko the "native" browser engine?
If anything, the native browser engine for Windows would be MSHTML from Internet Explorer.
All these kind of CPU level vulnerabilities are the same, they are only really "risky" if there is malicious software running in the computer in the first place.
The real problem is that these CPU-level vulnerabilities all break one of the core concepts of computers, which is process separation and virtual memory. If process separation is broken then all other levels of security become pointless.
While for desktops this isn't a huge problem (except when sometimes vulnerabilities might even be able to be exploited though browsers), this is a huge problem for servers, where the modern cloud usually has multiple users in virtual machines in a single server and a malicious user could steal information across virtual machines.