### Preflight Checklist
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* [x] I have read the [Contributing Guidelines](https://github.com/electron/electron/blob/master/CONTRIBUTING.md) for this project.
* [x] I agree to follow the [Code of Conduct](https://github.com/electron/electron/blob/master/CODE_OF_CONDUCT.md) that this project adheres to.
* [x] I have searched the issue tracker for an issue that matches the one I want to file, without success.

### Issue Details

* **Electron Version:** 7.1.2
* **Operating System:** Ubuntu 18.04 x64 
* **Last Known Working Electron version:** 6.x

### Expected Behavior
Setting the "ignore-connections-limit" commandLine switch for localhost should allow more than 4 connections to e.g. the gatsby development server, which limits it to that number. This is the behaviour in Electron@6

### Actual Behavior
The "ignore-connections-limit" seems to have no effect anymore in electron@7.

### To Reproduce

1. Run a gatsby local server, like https://github.com/gatsbyjs/gatsby-starter-hello-world
2. Launch this fiddle with electron@6, and see that all 6 iframes load the site: https://gist.github.com/Kilian/d35badc28200c3229b1028243433cf54
3. Launch the fiddle with electron@7, see that none of the iframes load anymore
4. Remove two iframes, launch with electron@7, see that the iframes now do load

### Note
Note that our documentation still mentions the flag as working: https://electronjs.org/docs/api/chrome-command-line-switches#--ignore-connections-limitdomains
shell/browser/net/proxying_url_loader_factory.cc
const net::HttpRequestHeaders& headers,
    OnBeforeSendHeadersCallback callback) {
  if (!current_request_uses_header_client_) {
    std::move(callback).Run(net::OK, base::nullopt);
    return;
  }
  request_.headers = headers;
  on_before_send_headers_callback_ = std::move(callback);
  ContinueToBeforeSendHeaders(net::OK);
}
void ProxyingURLLoaderFactory::InProgressRequest::OnHeadersReceived(
    const std::string& headers,
    const net::IPEndPoint& endpoint,
    OnHeadersReceivedCallback callback) {
  if (!current_request_uses_header_client_) {
    std::move(callback).Run(net::OK, base::nullopt, GURL());
    return;
  }
  on_headers_received_callback_ = std::move(callback);
  current_response_ = network::mojom::URLResponseHead::New();
  current_response_->headers =
      base::MakeRefCounted<net::HttpResponseHeaders>(headers);
  HandleResponseOrRedirectHeaders(
      base::BindOnce(&InProgressRequest::ContinueToHandleOverrideHeaders,
                     weak_factory_.GetWeakPtr()));
}
void ProxyingURLLoaderFactory::InProgressRequest::ContinueToBeforeSendHeaders(