My main motivation is that it is easier to type Maps than Objects-as-maps, but
using Objects as maps can have some odd behavior if you aren't careful, so I
think it is probably a better idea to use anyways for this data that might
be coming from a compromised child process.
This slightly alters the behavior of autofillFields because it modifies
the existing map in place rather than creating a new one from scratch,
but I think that is preferable because nothing seems to have an external
reference so there's no need to preserve the old map.
Differential Revision: https://phabricator.services.mozilla.com/D219361
a378e69577