1. OuterDocAccessible::ChildCount was previously Windows only. Implement it for all platforms.
2. OuterDocAccessible::ChildAt included code for Windows which isn't necessary because it calls AccessibleWrap::LocalChildAt, not OuterDocAccessible::LocalChildAt (where the Windows specific RemoteAccessibleWrap stuff is implemented).
3. LocalAccessible::ChildAtPoint previously expected LocalChildAt() to succeed if ChildCount > 1. As per 1), this is no longer true for OuterDocAccessibles containing a remote document. Adjust accordingly.
4. IndexInParent on a top level remote document was previously returning -1. Implement DocAccessibleParent::IndexInParent to fix this.
5. Doing 4) means that RemoteNext/PrevSibling broke for top level documents because they use IndexInParent, but there is no remote parent. Add a specific early return for documents there.
Differential Revision: https://phabricator.services.mozilla.com/D115045
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