On Windows, the only time the web process is shut down is if all WKPages are released. If this does not happen, the web process never gets shut down cleanly.
See also bug 42825, which is how I discovered this problem.
It's possible that this is a cause of bug 42785. See discussion in that bug for more details.
This seems no longer to be true (at least, once bug 43150 and bug 42826 are fixed, which is how I'm testing; otherwise there's an assertion on quit).