The XULSelectControlAccessible selection methods don't handle ARIA selection.
Therefore, if aria-multiselectable is set, use the base implementation of the selection retrieval methods.
We don't bother overriding the selection setting methods because implementations (e.g. browser tabs) don't support setting of aria-selected by the a11y engine and we still want to be able to set the primary selected item according to XUL.
Being able to retrieve multiple selection programmatically is far more important than being able to set it.
MozReview-Commit-ID: CmVp9KyieMY
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extra : rebase_source : e3fa93aad4726b322956babb5422dceebfa0fbb2
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