### 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.
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### 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
removed_headers.begin(), removed_headers.end());
    for (auto& set_header : set_headers) {
      std::string header_value;
      if (request_.headers.GetHeader(set_header, &header_value)) {
        pending_follow_redirect_params_->modified_headers.SetHeader(
            set_header, header_value);
      } else {
        NOTREACHED();
      }
    }
    if (target_loader_.is_bound()) {
      target_loader_->FollowRedirect(
          pending_follow_redirect_params_->removed_headers,
          pending_follow_redirect_params_->modified_headers,
          pending_follow_redirect_params_->new_url);
    }
    pending_follow_redirect_params_.reset();
  }
  if (proxied_client_binding_.is_bound())
    proxied_client_binding_.ResumeIncomingMethodCallProcessing();
  
  factory_->web_request_api()->OnSendHeaders(&info_.value(), request_,
                                             request_.headers);
  if (!current_request_uses_header_client_)
    ContinueToStartRequest(net::OK);
}
void ProxyingURLLoaderFactory::InProgressRequest::ContinueToStartRequest(
    int error_code) {
  if (error_code != net::OK) {
    OnRequestError(network::URLLoaderCompletionStatus(error_code));