Clients need the xml-roles object attribute to get access to the raw ARIA role for cases where Role() doesn't return sufficient information; e.g. landmarks. In most cases, we can just derive this from the role map index, which we already send. However, the role attribute is technically a list of tokens, so it could include multiple roles or roles unknown to Gecko.

This also changes LocalAccessible::Attributes to ignore the edge case of an empty role attribute when calculating xml-roles.
This is consistent with how we handle the role attribute elsewhere (see aria::GetRoleMapIndex) and makes it easier to handle this consistently between local and remote.

https://hg.mozilla.org/mozilla-central/rev/b0aa1d65dad5