The X11 backend uses EWMH's _NET_WM_PID to get the PID of an offending
client and kill its PID to force the client to terminate.
The Wayland backend is using a Wayland protocol error, but if the client
is hung, that will not be sufficient to kill the client.
Retrieve the client PID under Wayland using the Wayland client API
wl_client_get_credentials() and kill() the client the same way the X11
backend does.
https://bugzilla.gnome.org/show_bug.cgi?id=767464
c72efc90cb