The cast in InitWithNode is wrong. AsElement() asserts instead of
checking the flag, so we always pass an element (and if we didn't we'd
have type confusion problems). I audited the callers and we're fine.
Anyhow, always require an element, and add two convenience constructors
for C++ code.
Differential Revision: https://phabricator.services.mozilla.com/D73636
41fba10f7f