just paste into console and let the web app do it's thing, all messages will get logged.

chromecast-session-hijacking.js

const real = chrome.cast.requestSession;
chrome.cast.requestSession = (success, ...dontcare) => {
    real(session => {
        console.log('intercepted session!', session);
        for (let namespace of session.namespaces) {
            console.log('listening on', namespace.name);
            session.addMessageListener(namespace.name, (namespace, message) => console.log('<=', namespace, JSON.parse(message)));
            const realSend = session.sendMessage;
            session.sendMessage = (namespace, message, ...whatever) => {
                console.log('=>', namespace, message);
                realSend.bind(session)(namespace, message, ...whatever);
            }
        }

        //pass modified version back to app >:)
        success(session);
    }, ...dontcare)
}