If any district boundary is a street line or is determined, measured or fixed by or from a street line and such street shall be widened and such new width has been duly authorized and recorded on a duly filed map, then anything in this chapter to the contrary notwithstanding, the district boundary shall not be determined, measured or fixed by or from the new street line, but the rear district boundary shall remain as theretofore, and the depth and extent of the district shall be shortened and lessened accordingly.