### Preflight Checklist
<!-- Please ensure you've completed the following steps by replacing [ ] with [x]-->

* [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
mojo_base::BigBuffer data) {
  target_client_->OnReceiveCachedMetadata(std::move(data));
}
void ProxyingURLLoaderFactory::InProgressRequest::OnTransferSizeUpdated(
    int32_t transfer_size_diff) {
  target_client_->OnTransferSizeUpdated(transfer_size_diff);
}
void ProxyingURLLoaderFactory::InProgressRequest::OnStartLoadingResponseBody(
    mojo::ScopedDataPipeConsumerHandle body) {
  target_client_->OnStartLoadingResponseBody(std::move(body));
}
void ProxyingURLLoaderFactory::InProgressRequest::OnComplete(
    const network::URLLoaderCompletionStatus& status) {
  if (status.error_code != net::OK) {
    OnRequestError(status);
    return;
  }
  target_client_->OnComplete(status);
  factory_->web_request_api()->OnCompleted(&info_.value(), request_,
                                           status.error_code);
  
  factory_->RemoveRequest(network_service_request_id_, request_id_);
}
void ProxyingURLLoaderFactory::InProgressRequest::OnLoaderCreated(
    mojo::PendingReceiver<network::mojom::TrustedHeaderClient> receiver) {
  header_client_receiver_.Bind(std::move(receiver));
}
void ProxyingURLLoaderFactory::InProgressRequest::OnBeforeSendHeaders(