[chromium-discuss] Add support for the XDG Base Dir Spec for certificates dir.

[e***@gmail.com]

Hello!

I would like to code this change as follows: move the default dir from
$HOME/.pki/nssdb to ${XDG_DATA_HOME:-$HOME/.local/share}/pki/nssdb for
Linux installations only. I could add legacy support by checking if the
folder $HOME/.pki/nssdb already exists; it it does, then we use this path.
Otherwise, we use the new XDG path.

I have a prototype for this already working on my current branch. It is a
very simple change, as you can see on the attached diff.

I wanted to discuss a little bit this feature before submitting. What do
you guys think about this change?

There is something else to discuss about the certs dir location, but I need
to get this dealt first.