Tracked down as the cause of https://github.com/devtools-html/debugger.html/issues/6923. source-map-url-service allows callers to register subscribers, but in this case the console is actually calling `.subscribe` before the URL service has received the "source-updated" event that makes it aware of the script in the first place. To avoid this race condition, it seems like `source-updated` events should also be sure to trigger any subscribers that have been added for the file.

Pushed by lsmyth@mozilla.com: https://hg.mozilla.org/integration/autoland/rev/f3d51256c5d7 source-map-url-service should trigger subscribers when files matching them are detected. r=tromey