js: Use JS_Enumerate instead of iterator

JS_NewPropertyIterator() and its related functions are gone in mozjs38;
apparently they were broken anyway for indexed properties (i.e. those
with integer IDs rather than string IDs.)

Instead, we use JS_Enumerate() which often makes the code cleaner anyway.
Note that the JS::AutoIdArray does not root the jsids inside it, it just
manages the array wrapper.

https://bugzilla.gnome.org/show_bug.cgi?id=776966