In-process iframes get a DocAccessibleChild, even though they're not top level in their process.
However, the parent IAccessible on DocAccessibleChild (sent from the parent process) is only relevant for the top level.
Previously, we used the parent IAccessible on the DocAccessibleChild for *all* documents, resulting in accParent returning null for in-process iframe documents.
Instead, we need to use the base implementation in this case.
Differential Revision: https://phabricator.services.mozilla.com/D74180
21678aeb07