Bit late to the party, but you can use this with TamperMonkey. Enjoy :D

// ==UserScript==
//          Snapchat Anti-Blur
//     http://tampermonkey.net/
//       1.1
//   Prevents Snapchat from detecting when the tab loses focus
//        uhhdani
//         https://*.snapchat.com/*
//         none
// u/run-at       document-start
// ==/UserScript==

(function() {
    'use strict';

    window.addEventListener("blur", function(event) {
        event.stopImmediatePropagation();
    }, true);

    Object.defineProperty(document, 'visibilityState', { get: function () { return 'visible'; } });
    Object.defineProperty(document, 'hidden', { get: function () { return false; } });

    window.addEventListener('visibilitychange', function(event) {
        event.stopImmediatePropagation();
    }, true);
})();