Persist fully resolved icon URL
The resolved URL is passed to alert service as expected already, and this patch makes sure Notification object itself stores the resolved URL as the spec says.
Safari and Chrome already conform to the spec.
Removing test_notification_get as it's now covered by WPT and modifying notification_worker as WPT doesn't cover dedicated workers yet (as testdriver limitation makes it hard).
Differential Revision: https://phabricator.services.mozilla.com/D208132
bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1892705
gecko-commit: b98e368f9ae3cb97269a3973f08be17dda6b22ec
gecko-reviewers: asuth, hsingh