You can download the older deb
http://launchpadlibrarian.net/319181475/chromium-browser_58.0.3029.110-0ubuntu0.14.04.1176_armhf.deb
But it also requires the exact chromium-codecs-ffmpeg-extra package
I did somehting along the lines of this...
sudo apt remove chromium-browser
mkdir tempforinstall
cd tempforinstall
wget http://launchpadlibrarian.net/319181475/chromium-browser_58.0.3029.110-0ubuntu0.14.04.1176_armhf.deb
wget http://launchpadlibrarian.net/319181477/chromium-codecs-ffmpeg-extra_58.0.3029.110-0ubuntu0.14.04.1176_armhf.deb
sudo dpkg -i *.deb
sudo -s
echo chromium-browser hold | dpkg --set-selections
This removes the 59 version, downloads 58 versions, installs them and then locks the browser so it doesn't get upgraded. I'm sure someone could do this in less lines but it worked for me.
I don't get the Lock option in synaptic for some reason.
