Keystone is Google's auto-updater program. It updates not only Chrome but also Earth and other Google programs. It's a notorious resource hog and it tries very hard not to let you ever turn it off. If you manage to uninstall it, it will try even harder to reinstall itself the next time you run a Google app.

Keystone is malware made by Google. The incident this week was the first time it contained an actual destructive payload, but it's been malware for years.

Super short version: Keystone appears to be the Google auto updater service. It has a bug that causes it to unlink /var, and since that's sort of an important piece of the OS, its absence breaks stuff.