Ubuntu – Running script at shutdown and reboot to prevent Chromium session loss

I'm running Ubuntu 18.04.1 and I'm trying to create a script that shuts down Chromium properly before shutting down the system so that it doesn't give me "restore session" popup after booting. I figured I'll use killall to this and came up with
/home/shutdownscript.sh:

#!/bin/bash 
killall -HUP "chromium-browser --enable-pinch"

that seems to do the trick.

However, now I have a problem with actually running this script at shutdown or reboot.

First thing I tried was putting it in /etc/init.d as shutdownscript with chmod a+x and then symlinking it to rc0.d and rc6.d as K99shutdownscript and later K01ashutdownscript. However, that didn't work for me.

I thought maybe I should just create a new systemd service, so I created shutdownscriptnew.service in /etc/systemd/system with contents like this:

#!/bin/bash
### BEGIN INIT INFO
# Provides:          shutdownscript
# Required-Start:    
# Required-Stop:     
# Default-Start:     
# Default-Stop:      0 1 6
# Short-Description: Start daemon at boot time
# Description:       Enable service provided by daemon.
### END INIT INFO

[Unit]
Description=Saves Chromium session
Before=shutdown.target reboot.target halt.target

[Service]
Type=oneshot
RemainAfterExit=true
ExecStop=/home/istir/shutdownscript.sh

[Install]
WantedBy=multi-user.target

Then I ran systemctl start shutdownscriptnew.service and systemctl enable shutdownscriptnew.service but it still didn't work as intended.
However, when I run systemctl stop shutdownscriptnew.service it actually does what it should do and closes chromium properly so that after I open it again it doesn't want to restore pages.

Ubuntu 18.04.1 LTS
GNOME Shell 3.28.2
Chromium installed from Ubuntu's software application (chromium-browser)
Shutting down from GNOME taskbar.

I also tried just installing Google Chrome from google.com/chrome/ but it still wanted to restore pages after reboot.

I specifically want to be able to run script like this and not fix chromium because I have similar issue with Spotify – it doesn't save my last song when I shut down my PC and I'm hoping to do something similar to it.

I guess the problem is with shutdownscriptnew.service but I searched and searched and can't find anything that works for me.

Thanks for help!