[w3c/selection-api] Make setBaseAndExtent and other setter methods on Selection work with shadow DOM (#99)

Per https://github.com/w3c/webcomponents/issues/79, we should make `setBaseAndExtent` and other methods work with shadow DOM. Namely, when a node inside a shadow tree is given to `setBaseAndExtent`, it shouldn't throw or adjust the end points to be in the same tree.

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https://github.com/w3c/selection-api/issues/99