London-based Aegis Group today confirmed it has signed a definitive agreement to acquire Beijing Catch Stone Advertising, the big digital media and marketing services company based in the Chinese capital. Aegis said it would pay an initial $86 million for control of the agency, but that the final price could be nearly triple that -- $236 million -- if the shop achieves certain performance levels over the next four years. The acquisition comes just a little more than a week after Aegis itself agreed to be acquired by Japanese agency holding Dentsu for nearly $5 billion. That deal is subject to Aegis shareholder and regulatory approval. However, the parties expect that acquisition to close in the fourth quarter barring any unforeseen…