Allow missing OpenVPN pid file on shutdown

If the OpenVPN process has already exited, the pid file will
have been removed.  Allow this to fail silently.

BUG=chromium:377962
TEST=Run this test

Change-Id: I1fff3b7a17a862c88c60abea7c5a52a8d57e091b
Reviewed-on: https://chromium-review.googlesource.com/201669
Reviewed-by: mukesh agrawal <quiche@chromium.org>
Commit-Queue: Paul Stewart <pstew@chromium.org>
Tested-by: Paul Stewart <pstew@chromium.org>
diff --git a/client/cros/vpn_server.py b/client/cros/vpn_server.py
index 62452f9..10cf768 100644
--- a/client/cros/vpn_server.py
+++ b/client/cros/vpn_server.py
@@ -302,5 +302,5 @@
     def stop_server(self):
         """Start VPN server instance"""
         chroot = self._chroot
-        chroot.kill_pid_file(self.OPENVPN_PID_FILE)
+        chroot.kill_pid_file(self.OPENVPN_PID_FILE, missing_ok=True)
         chroot.shutdown()