### 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.h
: public network::mojom::URLLoaderFactory,
      public network::mojom::TrustedURLLoaderHeaderClient {
 public:
  class InProgressRequest : public network::mojom::URLLoader,
                            public network::mojom::URLLoaderClient,
                            public network::mojom::TrustedHeaderClient {
   public:
    InProgressRequest(
        ProxyingURLLoaderFactory* factory,
        int64_t web_request_id,
        int32_t routing_id,
        int32_t network_service_request_id,
        uint32_t options,
        const network::ResourceRequest& request,
        const net::MutableNetworkTrafficAnnotationTag& traffic_annotation,
        network::mojom::URLLoaderRequest loader_request,
        network::mojom::URLLoaderClientPtr client);
    ~InProgressRequest() override;
    void Restart();
    
    void FollowRedirect(const std::vector<std::string>& removed_headers,
                        const net::HttpRequestHeaders& modified_headers,
                        const base::Optional<GURL>& new_url) override;
    void SetPriority(net::RequestPriority priority,
                     int32_t intra_priority_value) override;
    void PauseReadingBodyFromNet() override;
    void ResumeReadingBodyFromNet() override;