merge_commit_cc9dc6bb113ac7e04aee6b8e4157fbe8eb2d731c

"[cookie-store] Deflake service worker registration"
"Instead of attempting to unregister previously installed service workers
at the beginning of a test, use add_completion_callback to unregister
them at test completion. This will deflake the tests when they are run
multiple times without restarts (--rerun).

The alternative is to do what many tests in service-workers do:
unregister service workers at the start, but wait for newly installed
service workers to become activated and use registration.active
instead of registration.installing, which is a bit more complex.
"