I'm not sure what these APIs are exactly and why they're there, but Firefox also does something similar. It has special APIs available only to Mozilla and/or Firefox domains, for things like installing extensions, or helping with first-run experience.
A blog post about it was shared here on Hacker news <12 months ago, but I'm having trouble finding it...
A blog post about it was shared here on Hacker news <12 months ago, but I'm having trouble finding it...