I loaded a Chrome extension in Chrome using load unpacked in developer mode. I know how to debug inside Devtools.
How does one develop and debug an extension using WebStorm where WebStorm's breakpoints in the extension's Javascript get hit?
No special support for debugging Chrome extensions is currently provided, see WEB-9708.
The IDE can attach a debugger to a running Chrome instance started with --remote-debugging-port option using Attach to Node.js\Chrome run configuration; see if it works for you