This misses the final, most important part of actually creating a
service worker object and sending the script over to it in a WebWorker
process.
(cherry picked from commit 85541f1e763f2db63a235f57a29026331138a29e)
This approach will need some rework to be properly handled at the user
agent level instead of per renderer process, but it's a start.
(cherry picked from commit 7faebb2702a2a19fc401fde1856a0d90861d2904)
