Section 28. If a child, or the issue of a child, omitted in the will takes under section twenty a portion of the estate of a testator, such portion of the estate shall, for the purposes of the two preceding sections and section thirty, be considered as if it had been devised or bequeathed to such child or other descendant; and he shall contribute with the devisees and legatees, and be entitled to claim contribution from them, as before provided.