Hi, This is what I did and it's working fine on my end:

odoo.define('mymodule.AbstractWebClient', function (require) {
"use strict";
/** 
* AbstractWebClient Override Method for Page Title using Company Name 
*/

var session = require('web.session');
var AbstractWebClient = require('web.AbstractWebClient');

AbstractWebClient.include({
    _title_changed:function () {
        this._super();
        var company_name = session.user_companies.current_company[1];
        document.title = company_name;    
    },
});

return AbstractWebClient;
});

Hi, 
Show us the declaration of your js file.
Then add console.log('==========') after var session line to check if odoo reads the file
then add another one inside yuour fnution to check if it's printed too.

waiting for your reply.

Regards.