editing.nodes: Utilize |Node.prototype.contains()| in |editing.nodes.isDescendantOf()|.
Revert "editing.nodes: Utilize |Node.prototype.contains()| in |editing.nodes.isDescendantOf()|."

This reverts commit 1961e630afd45095b231537199bd85d40427af94.
Because of closure compiler have wrong externs for |Node.prototype.contains()|.