* Electron version: 1.6.1
* Operating system: macOS 10.12.3, Windows 7 64bit

### Expected behavior

Should emit an error on the request object.

### Actual behavior

<img width="420" alt="image" src="https://cloud.githubusercontent.com/assets/99662/23713890/4e9b8f52-041f-11e7-9dfb-e8408bb58670.png">

An uncatchable error happens.

### How to reproduce
```bash
git clone https://github.com/arantes555/electron-net-encoding-testcase.git
cd electron-net-encoding-testcase
npm install
npm start
```

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,
                           bool is_last) {
  DCHECK_CURRENTLY_ON(content::BrowserThread::UI);
  return content::BrowserThread::PostTask(
      content::BrowserThread::IO, FROM_HERE,
      base::Bind(&AtomURLRequest::DoWriteBuffer, this, buffer, is_last));
}
void AtomURLRequest::SetChunkedUpload(bool is_chunked_upload) {
  DCHECK_CURRENTLY_ON(content::BrowserThread::UI);
  
  
  
  is_chunked_upload_ = is_chunked_upload;
}
void AtomURLRequest::Cancel() {
  DCHECK_CURRENTLY_ON(content::BrowserThread::UI);
  content::BrowserThread::PostTask(content::BrowserThread::IO, FROM_HERE,
                                   base::Bind(&AtomURLRequest::DoCancel, this));
}
void AtomURLRequest::SetExtraHeader(const std::string& name,