### 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 OnReceiveResponse(network::mojom::URLResponseHeadPtr head) override;
    void OnReceiveRedirect(const net::RedirectInfo& redirect_info,
                           network::mojom::URLResponseHeadPtr head) override;
    void OnUploadProgress(int64_t current_position,
                          int64_t total_size,
                          OnUploadProgressCallback callback) override;
    void OnReceiveCachedMetadata(mojo_base::BigBuffer data) override;
    void OnTransferSizeUpdated(int32_t transfer_size_diff) override;
    void OnStartLoadingResponseBody(
        mojo::ScopedDataPipeConsumerHandle body) override;
    void OnComplete(const network::URLLoaderCompletionStatus& status) override;
    void OnLoaderCreated(
        mojo::PendingReceiver<network::mojom::TrustedHeaderClient> receiver);
    
    void OnBeforeSendHeaders(const net::HttpRequestHeaders& headers,
                             OnBeforeSendHeadersCallback callback) override;
    void OnHeadersReceived(const std::string& headers,
                           const net::IPEndPoint& endpoint,
                           OnHeadersReceivedCallback callback) override;
   private:
    
    void UpdateRequestInfo();
    void RestartInternal();
    void ContinueToBeforeSendHeaders(int error_code);
    void ContinueToSendHeaders(const std::set<std::string>& removed_headers,
                               const std::set<std::string>& set_headers,
                               int error_code);
    void ContinueToStartRequest(int error_code);
    void ContinueToHandleOverrideHeaders(int error_code);
    void ContinueToResponseStarted(int error_code);