### Preflight Checklist

- [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](https://www.github.com/electron/electron/issues) for a feature request that matches the one I want to file, without success.

### Problem Description

We need to load a custom error page when the certificate is invalid but without "isMainFrame" property we don't know when to load the custom error page using webContents.loadURL()

### Proposed Solution

webContents.on("certificate-error", ((event, url, error, certificate, callback, isMainFrame)

### Alternatives Considered

Not sure

### Additional Information

Not sure
shell/browser/api/electron_api_app.cc
#include "chrome/common/chrome_features.h"
#include "chrome/common/chrome_paths.h"
#include "content/browser/gpu/compositor_util.h"        
#include "content/browser/gpu/gpu_data_manager_impl.h"  
#include "content/public/browser/browser_accessibility_state.h"
#include "content/public/browser/browser_child_process_host.h"
#include "content/public/browser/child_process_data.h"
#include "content/public/browser/client_certificate_delegate.h"
#include "content/public/browser/gpu_data_manager.h"
#include "content/public/browser/network_service_instance.h"
#include "content/public/browser/render_frame_host.h"
#include "content/public/common/content_switches.h"
#include "media/audio/audio_manager.h"
#include "net/dns/public/util.h"
#include "net/ssl/client_cert_identity.h"
#include "net/ssl/ssl_cert_request_info.h"
#include "net/ssl/ssl_private_key.h"
#include "sandbox/policy/switches.h"
#include "services/network/network_service.h"
#include "shell/app/command_line_args.h"
#include "shell/browser/api/electron_api_menu.h"
#include "shell/browser/api/electron_api_session.h"
#include "shell/browser/api/electron_api_web_contents.h"
#include "shell/browser/api/gpuinfo_manager.h"
#include "shell/browser/electron_browser_context.h"
#include "shell/browser/electron_browser_main_parts.h"
#include "shell/browser/javascript_environment.h"
#include "shell/browser/login_handler.h"
#include "shell/browser/relauncher.h"
#include "shell/common/application_info.h"