### 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

Hi,

My team is trying to support universal linking in our application. Currently, in order to receive a handoff event from the browser, the application needs to listen to `continue-activity` event which provides `type` and `userInfo`. As per the Apple documentation [here](https://developer.apple.com/library/archive/documentation/UserExperience/Conceptual/Handoff/AdoptingHandoff/AdoptingHandoff.html#//apple_ref/doc/uid/TP40014338-CH2-SW10), the browser sends the app an `NSUserActivity` object with an `activityType` value of `NSUserActivityTypeBrowsingWeb`. The `webpageURL` property contains the URL the user was visiting, while the `userInfo` dictionary is empty. 

To provide a smooth handoff our application needs to know the URL the user is navigating from. Therefore, it would be great if the `webpageURL` parameter is exposed in `continue-activity` event. 


### Proposed Solution

The `NSUserActivity` object contains the `webpageURL` property which is set during the handoff process. 
I don't have any experience with Objective C but I believe the `webpageURL` property on the `userActivity` object can be allowed to pass [here](https://github.com/electron/electron/blob/da2baabb96161ea08a15820451f9374d63b4d678/shell/browser/mac/electron_application_delegate.mm#L161) to make it accessible.
https://github.com/electron/electron/blob/da2baabb96161ea08a15820451f9374d63b4d678/shell/browser/mac/electron_application_delegate.mm#L161


### Alternatives Considered

-

### Additional Information

The application needs to know the URL from where the user is navigating to provide a customized handoff experience. 
shell/browser/api/electron_api_app.h
#if defined(OS_MAC)
  void OnWillContinueUserActivity(bool* prevent_default,
                                  const std::string& type) override;
  void OnDidFailToContinueUserActivity(const std::string& type,
                                       const std::string& error) override;
  void OnContinueUserActivity(bool* prevent_default,
                              const std::string& type,
                              const base::DictionaryValue& user_info) override;
  void OnUserActivityWasContinued(
      const std::string& type,
      const base::DictionaryValue& user_info) override;
  void OnUpdateUserActivityState(
      bool* prevent_default,
      const std::string& type,
      const base::DictionaryValue& user_info) override;
  void OnNewWindowForTab() override;
  void OnDidBecomeActive() override;
#endif
  
  void AllowCertificateError(
      content::WebContents* web_contents,
      int cert_error,
      const net::SSLInfo& ssl_info,
      const GURL& request_url,
      bool is_main_frame_request,
      bool strict_enforcement,
      base::OnceCallback<void(content::CertificateRequestResultType)> callback)
      override;
  base::OnceClosure SelectClientCertificate(
      content::WebContents* web_contents,