chromium is a core18-based snap, you’ll need to build it on bionic.
That said, if all you need is to add a plug, I’d recommend not rebuild the whole thing: compiling chromium takes many hours, even on a beefy machine.
You can take an existing snap, unpack it, add the plug to the snap’s metadata, and repack it like so:
snap download chromium
unsquashfs chromium_*.snap
# edit squashfs-root/meta/snap.yaml to add the plug and save
snapcraft pack squashfs-root