They didn't add ARKit support to Chrome. ARKit only runs on iOS. Chrome isn't allowed to run on iOS. (The iOS Chrome app is just WKWebView with a Chrome branding wrapper, plus a few features bolted onto the side.) But even if somebody did build the real Chrome browser on iOS, Apple wouldn't allow anyone to publish it on their app store.
On iOS, WebARonARKit simply uses WKWebView's standard method of sending messages back and forth to the browser:
My question is why did they have to make a custom Chrome build on Android, to simply integrate the browser with a library like they did on iOS (WKWebView <=> ARKit)? Why didn't they use the off-the-shelf WebView browser component for Android instead of building their own monolithic custom Chrome app, just to integrate the browser with an Android library (WebView <=> arcore-android-sdk)?
[1]: https://developers.google.com/ar/develop/web/getting-started