US media giants CBS and Viacom, both controlled by the Redstone family, have agreed to combine in an all-stock merger to form ViacomCBS. CBS has a market value of $18 billion and Viacom about $11.7 billion. Shari Redstone, the daughter of billionaire Sumner Redstone, would become chairwoman of ViacomCBS, which will comprise brands like MTV, Comedy Central and Paramount Pictures.