When plats or blocks of farm units have been or are filed under the provisions of chapter 89.12 RCW which contain a system of county roads, or when a supplemental plat of a system of county roads to serve such a plat is filed in connection therewith, the filing period and formal approval by the board of county commissioners shall constitute establishment as county roads: PROVIDED, That the board of county commissioners have obtained the individual rights-of-way by deed or as otherwise provided by law.