Web research firm comScore, Inc. said Tuesday that it has agreed to buy film and TV viewership tracker Rentrak Corp. in an all-stock deal. Under the deal, which has been approved by the Boards of Directors of both companies, each share of Rentrak will be converted into the right to receive 1.15 shares of comScore. comScore shareholders are expected to own about 66.5% and Rentrak shareholders are expected to own about 33.5% of the combined company.