This is very similar to what I fought at my current place of work. I'm sure you've also found something like --proxy-server=http://proxy.example.com:8080
for the desktop file. This works, but I do not believe you can set a login there.
A few suggestions:
Does your proxy use Active Directory logins? Install cntlm
and
configure it, and then configure chrome to use localhost:3128 when
it's running.
Consider installing the foxyproxy plugin for chrome.
If it's a global proxy for a corporate environment and you have a
supported desktop environment, you can probably set it similarly in
the desktop environment settings and chrome will pick it up. GNOME
shell has built-ins for this, however if I remember correctly, you
need to use gconf-editor to get to the username and password bit.
Ultimately, I chose cntlm, since I can use it for my package manager (dnf on fedora), Firefox, and numerous other internet applications.