Bug 1422201 changed GetIAccessibleFor so it only handles remote ids when called on the root accessible.
However, this breaks webextension popup documents.
These popups have their own HWND, so the root accessible of that HWND needs to handle accChild for ids in remote documents within that HWND.
Therefore, expand the restriction to cover the root accessible of any HWND, not just the main HWND.
MozReview-Commit-ID: 69v4XSeQLcS
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extra : rebase_source : bf5c5e0a475424b2276800d1a10ab1a46c136bd6
47040c9214