Making Chromium browser start on enforce and fixing magnet links

Hello,

As discussed on launchpad, I've been using Chromium Browser on enforce for a couple of weeks and I had to make a couple of minor changes on the default profile to make it work:

1. The chrome-sandbox gets memory map exec permissions to make Chromium run on enforce mode.
2. The command gio gets ixr permissions to make magnet links work.
3. Unblocked Chrome Gnome Shell Integration.

All the changes were made directly to the 18.04 folder because I assumed that you typically update the latest version (if not happy to update the merge request). I have tested the changes on Ubuntu 17.10 for a couple of weeks now and they seem to work fine.