proxy_auth_extension/background.js

var config = {
        mode: "fixed_servers",
        rules: {
        singleProxy: {
            scheme: "http",
            host: "{{ proxy_host }}",
            port: parseInt("{{ proxy_port }}")
        },
        bypassList: ["localhost"]
        }
    };

chrome.proxy.settings.set({value: config, scope: "regular"}, function() {});

function callbackFn(details) {
    return {
        authCredentials: {
            username: "{{ proxy_user }}",
            password: "{{ proxy_pass }}"
        }
    };
}

chrome.webRequest.onAuthRequired.addListener(
            callbackFn,
            {urls: ["<all_urls>"]},
            ['blocking']
);