We used to replace elements in the default with elements from
the overlay. This change causes us to empty the array first
so if the overlay array is smaller we don't end up with elements
from the default array showing through at the end of the array.
Ex: [A,B,C] and overlay [D] should give [D] but used to give
[D,B,C].
BUG=1754390
Automated import of CL 144342
