static getRegistry_() {
    const override = shaka.offline.StorageMuxer.override_;
    const registry = shaka.offline.StorageMuxer.registry_;

    if (COMPILED) {
      return registry;
    } else {
      return override || registry;
    }
  }
Get the registry. If the support has been disabled, this will always
an empty registry. Reading should always be done via |getRegistry_|.

@return {!Map<string, function(): shaka.extern.StorageMechanism>}
@private