* Electron version: 1.7.9
* Operating system: macOS 10.12.6

### Expected behavior

Often Electron apps want to get in front of `openExternal` calls to prevent them, or display a "are you sure you want to open" dialog for possibly unsafe url schemes like `ssh://`. At the moment, you can either blanket deny these or allow all for a give host via a `permissionRequestHandler`. It would be helpful for the permission handler to be given the url that the window is attempting to navigate to better determine the action it should take.


```
 $ git clone git@github.com:gerges/electron-quick-start.git -b  feat/set-permission-handler-openexternal-url
 $ npm install
 $ npm run start
```
atom/browser/atom_permission_manager.h
protected:
  void OnPermissionResponse(int request_id,
                            int permission_id,
                            blink::mojom::PermissionStatus status);
  
  void CancelPermissionRequest(int request_id) override;
  void ResetPermission(content::PermissionType permission,
                       const GURL& requesting_origin,
                       const GURL& embedding_origin) override;
  blink::mojom::PermissionStatus GetPermissionStatus(
      content::PermissionType permission,
      const GURL& requesting_origin,
      const GURL& embedding_origin) override;
  int SubscribePermissionStatusChange(
      content::PermissionType permission,
      const GURL& requesting_origin,
      const GURL& embedding_origin,
      const base::Callback<void(blink::mojom::PermissionStatus)>& callback)
      override;
  void UnsubscribePermissionStatusChange(int subscription_id) override;
 private:
  class PendingRequest;
  using PendingRequestsMap = IDMap<std::unique_ptr<PendingRequest>>;
  RequestHandler request_handler_;
  PendingRequestsMap pending_requests_;
  DISALLOW_COPY_AND_ASSIGN(AtomPermissionManager);
};
}  
#endif