* 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/web_contents_permission_helper.h
POINTER_LOCK = static_cast<int>(content::PermissionType::NUM) + 1,
    FULLSCREEN,
    OPEN_EXTERNAL,
  };
  void RequestFullscreenPermission(
      const base::Callback<void(bool)>& callback);
  void RequestMediaAccessPermission(
      const content::MediaStreamRequest& request,
      const content::MediaResponseCallback& callback);
  void RequestWebNotificationPermission(
      const base::Callback<void(bool)>& callback);
  void RequestPointerLockPermission(bool user_gesture);
  void RequestOpenExternalPermission(
      const base::Callback<void(bool)>& callback,
      bool user_gesture);
 private:
  explicit WebContentsPermissionHelper(content::WebContents* web_contents);
  friend class content::WebContentsUserData<WebContentsPermissionHelper>;
  void RequestPermission(
      content::PermissionType permission,
      const base::Callback<void(bool)>& callback,
      bool user_gesture = false);
  content::WebContents* web_contents_;
  DISALLOW_COPY_AND_ASSIGN(WebContentsPermissionHelper);
};
}  
#endif