Do not coerce the javascript: return value to a string.
Update the code to follow the spec change:
whatwg/html@9997cd9
Differential Revision: https://phabricator.services.mozilla.com/D229475
bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1931058
gecko-commit: eb47c19459dd8a75db1b12a09c0bdc013679e18e
gecko-reviewers: smaug