WPP -- already the largest digital player on Madison Avenue by a wide margin -- just got bigger, expanding the footprint of its Possible Worldwide unit with the acquisition of one of the U.K.’s fastest-growing digital shops, Fortune Cookie. Terms of the deal were not disclosed, but FC Founder-CEO Justin Cooke will become the U.K. CEO of Possible, which already is one of WPP’s largest digital marketing services networks with 26 offices worldwide, and a roster including ATs dominance as the biggest and fastest-growing digital agency in Eastern Europe. The acquisition also gives Possible local offices in Sydney and Melbourne, Australia. FC, which was founded in 1997, works across all digital media platforms, and has a roster…