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Previously, the LocalAccessible implementation of ISimpleDOM allowed clients to walk to DOM nodes which were not included in the accessibility tree. Thus, it was necessary for sdnAccessible to be constructed with either an Accessible or a DOM node. Now that this functionality has been removed, we no longer need to support construction with a DOM node. This allows other code such as QueryInterface to be simplified as well, since an sdnAccessible now always has a corresponding MsaaAccessible. Differential Revision: https://phabricator.services.mozilla.com/D241806