Sadly, while WebRTC is "supported" by iOS 11, it is disabled in web apps added to the home screen, as well as any embedded web views like UIWebView or WKWebView.

Which leads to the wonderful UX that if a user adds your website that uses WebRTC to the homescreen, you need to catch a TypeError from getUserMedia being undefined, and display a modal to them telling them to delete the homescreen shortcut and always type in the URL if they want to use the website.