Walt Disney Co. and billionaire Mukesh Ambani’s conglomerate have signed a binding pact to merge their media operations in India, creating a sector behemoth valued at $8.5 billion in one of the world’s fastest-growing entertainment markets. The US media company will control 36.84%, while Ambani-led Reliance Industries Ltd. will own 16.34% in the joint venture, according to a Reliance statement on Wednesday.