AT&T and Time Warner on Saturday announced they had reached a merger deal, setting off alarm bells across the industry. AT&T has agreed to buy Time Warner for $85.4 billion. At least a year’s worth of regulatory scrutiny, public hearings and political maneuvering is expected. The deal, which followed days of speculation and tightly held negotiations, essentially would create the industry’s first vertically integrated mobile provider of premium cable. AT&T is already the largest pay-TV provider and one of the biggest telecoms in the world.