* Electron version: 1.8.2-beta.3
* Operating system: MacOS 10.12

### Expected behavior

File downloads from custom protocols should complete without issue.

### Actual behavior

Before even triggering the custom protocol handler, the download-item emits 'done' with a state of 'interrupted'. In my demo linked below, I use the new registerStreamProtocol, but I was able to reproduce using registerHttpProtocol as well.

### How to reproduce

Use https://github.com/pfrazee/electron-download-interrupt-bug

atom/browser/net/atom_url_request_job_factory.cc
AtomURLRequestJobFactory::~AtomURLRequestJobFactory() {
  Clear();
}
bool AtomURLRequestJobFactory::SetProtocolHandler(
    const std::string& scheme,
    std::unique_ptr<ProtocolHandler> protocol_handler) {
  if (!protocol_handler) {
    auto it = protocol_handler_map_.find(scheme);
    if (it == protocol_handler_map_.end())
      return false;
    delete it->second;
    protocol_handler_map_.erase(it);
    return true;
  }
  if (base::ContainsKey(protocol_handler_map_, scheme))
    return false;
  protocol_handler_map_[scheme] = protocol_handler.release();
  return true;
}
bool AtomURLRequestJobFactory::InterceptProtocol(