8ab709c677
Its only real purpose is to encapsulate a download ID, but it doesn't actually save any code, and arguably makes it more complicated by having multiple lists of Downloader instances, indexed by key, and also because it's another (undocumented) class to understand. This CL retains the multiple Downloader lists, but they will be flattened in a child CL. PiperOrigin-RevId: 308838026