Sorry actually it's not an extension, it's a "controller". It attaches to the debugging port of your Chrome instance.

I tried implementing it as an extension but there's a lot of (undocumented) differences to using CRDP [0] over websocket and over chrome.debugger extension API, so for now I gave up on the extension part of it.

[0]: https://chromedevtools.github.io/devtools-protocol/

[1]: https://developer.chrome.com/extensions/debugger