### 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.
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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.h
void ContinueToBeforeRedirect(const net::RedirectInfo& redirect_info,
                                  int error_code);
    void HandleBeforeRequestRedirect();
    void HandleResponseOrRedirectHeaders(
        net::CompletionOnceCallback continuation);
    void OnRequestError(const network::URLLoaderCompletionStatus& status);
    ProxyingURLLoaderFactory* factory_;
    network::ResourceRequest request_;
    const base::Optional<url::Origin> original_initiator_;
    const uint64_t request_id_;
    const int32_t routing_id_;
    const int32_t network_service_request_id_;
    const uint32_t options_;
    const net::MutableNetworkTrafficAnnotationTag traffic_annotation_;
    mojo::Binding<network::mojom::URLLoader> proxied_loader_binding_;
    network::mojom::URLLoaderClientPtr target_client_;
    base::Optional<extensions::WebRequestInfo> info_;
    network::mojom::URLResponseHeadPtr current_response_;
    scoped_refptr<net::HttpResponseHeaders> override_headers_;
    GURL redirect_url_;
    mojo::Binding<network::mojom::URLLoaderClient> proxied_client_binding_;
    network::mojom::URLLoaderPtr target_loader_;
    bool request_completed_ = false;
    
    
    
    
    
    
    bool has_any_extra_headers_listeners_ = false;