### 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
WebRequestAPI* web_request_api,
    const HandlersMap& intercepted_handlers,
    content::BrowserContext* browser_context,
    int render_process_id,
    network::mojom::URLLoaderFactoryRequest loader_request,
    mojo::PendingRemote<network::mojom::URLLoaderFactory> target_factory_remote,
    mojo::PendingReceiver<network::mojom::TrustedURLLoaderHeaderClient>
        header_client_receiver,
    content::ContentBrowserClient::URLLoaderFactoryType loader_factory_type)
    : web_request_api_(web_request_api),
      intercepted_handlers_(intercepted_handlers),
      browser_context_(browser_context),
      render_process_id_(render_process_id),
      loader_factory_type_(loader_factory_type) {
  target_factory_.Bind(std::move(target_factory_remote));
  target_factory_.set_disconnect_handler(base::BindOnce(
      &ProxyingURLLoaderFactory::OnTargetFactoryError, base::Unretained(this)));
  proxy_receivers_.Add(this, std::move(loader_request));
  proxy_receivers_.set_disconnect_handler(base::BindRepeating(
      &ProxyingURLLoaderFactory::OnProxyBindingError, base::Unretained(this)));
  if (header_client_receiver)
    url_loader_header_client_receiver_.Bind(std::move(header_client_receiver));
}
ProxyingURLLoaderFactory::~ProxyingURLLoaderFactory() = default;