Tests that User-Agent override works for requests from Service Workers.

navigated to http://localhost:8000/devtools/service-workers/resources/sw-return-useragent.php
user-agent: Mozilla/5.0 (Overridden User Agent)
awaited service worker target created
navigated to http://127.0.0.1:8000

Stopped worker and awaited target removal
navigated to http://localhost:8000/devtools/service-workers/resources/sw-return-useragent.php
user-agent: Mozilla/5.0 (Overridden User Agent)