The agreement between Baidu and One-Stop China, a joint venture between the Universal Music Group, the Warner Music Group and Sony BMG, will shut down access to a vast amount of pirated music and promises to broadly reshape the way China’s 450 million Web users gain access to online music. The country has long been a haven for pirated content. Baidu has been one of the chief conduits to it, much to the consternation of record labels, publishers and artists both here and abroad.