Items under non-tabbable shadow-roots should not be tabbable

[Eden-Mironi-Evinced]

The tabbable library identifies elements that are usually tabbable as tabbable even though that in cases the shadow root has attribute of tabIndex="-1" it should remove all of its children from the tabbing sequence.
In this case:

both of the buttons should not get a true value for the isTabbable function.

[stefcameron]

Thanks for bringing this to my attention!