* 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.cc
DCHECK_CURRENTLY_ON(content::BrowserThread::IO);
  request_.reset();
  if (request_context_getter_) {
    request_context_getter_->RemoveObserver(this);
    request_context_getter_ = nullptr;
  }
}
bool AtomURLRequest::Write(scoped_refptr<const net::IOBufferWithSize> buffer,